Remove duplicated content in concepts and reference

We currently have quite some contents duplicated between the concepts
and the reference section, which is not good. This PR is an effort to
remove the duplicated content from the two pages while providing links
for readers to check the reference easily.

Author: tengqm

content/en/docs/concepts/overview/kubernetes-api.md

@@ -16,29 +16,22 @@ card:
 
 The core of Kubernetes' {{< glossary_tooltip text="control plane" term_id="control-plane" >}}
 is the {{< glossary_tooltip text="API server" term_id="kube-apiserver" >}}. The API server
-exposes an HTTP API that lets end users, different parts of your cluster, and external components
-communicate with one another.
+exposes an HTTP API that lets end users, different parts of your cluster, and
+external components communicate with one another.
 
 The Kubernetes API lets you query and manipulate the state of objects in the Kubernetes API
 (for example: Pods, Namespaces, ConfigMaps, and Events).
 
-API endpoints, resource types and samples are described in the [API Reference](/docs/reference/kubernetes-api/).
+Most operations can be performed through the
+[kubectl](/docs/reference/kubectl/overview/) command-line interface or other
+command-line tools, such as
+[kubeadm](/docs/reference/setup-tools/kubeadm/kubeadm/), which in turn use the
+API. However, you can also access the API directly using REST calls.
 
-<!-- body -->
-
-## API changes
+Consider using one of the [client libraries](/docs/reference/using-api/client-libraries/)
+if you are writing an application using the Kubernetes API.
 
-Any system that is successful needs to grow and change as new use cases emerge or existing ones change.
-Therefore, Kubernetes has design features to allow the Kubernetes API to continuously change and grow.
-The Kubernetes project aims to _not_ break compatibility with existing clients, and to maintain that
-compatibility for a length of time so that other projects have an opportunity to adapt.
-
-In general, new API resources and new resource fields can be added often and frequently.
-Elimination of resources or fields requires following the
-[API deprecation policy](/docs/reference/using-api/deprecation-policy/).
-
-What constitutes a compatible change, and how to change the API, are detailed in
-[API changes](https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#readme).
+<!-- body -->
 
 ## OpenAPI specification {#api-specification}
 
@@ -78,95 +71,58 @@ You can request the response format using request headers as follows:
   <caption>Valid request header values for OpenAPI v2 queries</caption>
 </table>
 
-Kubernetes implements an alternative Protobuf based serialization format for the API that is primarily intended for intra-cluster communication, documented in the [design proposal](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/api-machinery/protobuf.md) and the IDL files for each schema are located in the Go packages that define the API objects.
-
-## API versioning
-
-To make it easier to eliminate fields or restructure resource representations, Kubernetes supports
-multiple API versions, each at a different API path, such as `/api/v1` or
-`/apis/rbac.authorization.k8s.io/v1alpha1`.
-
-Versioning is done at the API level rather than at the resource or field level to ensure that the
-API presents a clear, consistent view of system resources and behavior, and to enable controlling
-access to end-of-life and/or experimental APIs.
+Kubernetes implements an alternative Protobuf based serialization format that
+is primarily intended for intra-cluster communication. For more information
+about this format, see the [Kubernetes Protobuf serialization](https://github.com/kubernetes/community/blob/master/contributors/design-proposals/api-machinery/protobuf.md) design proposal and the
+Interface Definition Language (IDL) files for each schema located in the Go
+packages that define the API objects.
 
-The JSON and Protobuf serialization schemas follow the same guidelines for schema changes - all descriptions below cover both formats.
-
-Note that API versioning and Software versioning are only indirectly related.  The
-[Kubernetes Release Versioning](https://git.k8s.io/community/contributors/design-proposals/release/versioning.md)
-proposal describes the relationship between API versioning and software versioning.
-
-Different API versions imply different levels of stability and support.  The criteria for each level are described
-in more detail in the
-[API Changes](https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#alpha-beta-and-stable-versions)
-documentation.  They are summarized here:
-
-- Alpha level:
-  - The version names contain `alpha` (e.g. `v1alpha1`).
-  - May be buggy.  Enabling the feature may expose bugs.  Disabled by default.
-  - Support for feature may be dropped at any time without notice.
-  - The API may change in incompatible ways in a later software release without notice.
-  - Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.
-- Beta level:
-  - The version names contain `beta` (e.g. `v2beta3`).
-  - Code is well tested.  Enabling the feature is considered safe.  Enabled by default.
-  - Support for the overall feature will not be dropped, though details may change.
-  - The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release.  When this happens,
-    we will provide instructions for migrating to the next version.  This may require deleting, editing, and re-creating
-    API objects.  The editing process may require some thought.   This may require downtime for applications that rely on the feature.
-  - Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases.  If you have
-    multiple clusters which can be upgraded independently, you may be able to relax this restriction.
-  - **Please do try our beta features and give feedback on them!  Once they exit beta, it may not be practical for us to make more changes.**
-- Stable level:
-  - The version name is `vX` where `X` is an integer.
-  - Stable versions of features will appear in released software for many subsequent versions.
-
-## API groups
-
-To make it easier to extend its API, Kubernetes implements [*API groups*](https://git.k8s.io/community/contributors/design-proposals/api-machinery/api-group.md).
-The API group is specified in a REST path and in the `apiVersion` field of a serialized object.
-
-There are several API groups in a cluster:
+## API changes
 
-1. The *core* group, also referred to as the *legacy* group, is at the REST path `/api/v1` and uses `apiVersion: v1`.
+Any system that is successful needs to grow and change as new use cases emerge or existing ones change.
+Therefore, Kubernetes has designed its features to allow the Kubernetes API to continuously change and grow.
+The Kubernetes project aims to _not_ break compatibility with existing clients, and to maintain that
+compatibility for a length of time so that other projects have an opportunity to adapt.
 
-1. *Named* groups are at REST path `/apis/$GROUP_NAME/$VERSION`, and use `apiVersion: $GROUP_NAME/$VERSION`
-   (e.g. `apiVersion: batch/v1`). The Kubernetes [API reference](/docs/reference/kubernetes-api/) has a
-   full list of available API groups.
+In general, new API resources and new resource fields can be added often and frequently.
+Elimination of resources or fields requires following the
+[API deprecation policy](/docs/reference/using-api/deprecation-policy/).
 
-There are two paths to extending the API with [custom resources](/docs/concepts/extend-kubernetes/api-extension/custom-resources/):
+What constitutes a compatible change, and how to change the API, are detailed in
+[API changes](https://git.k8s.io/community/contributors/devel/sig-architecture/api_changes.md#readme).
 
-1. [CustomResourceDefinition](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/)
-   lets you declaratively define how the API server should provide your chosen resource API.
-1. You can also [implement your own extension API server](/docs/tasks/extend-kubernetes/setup-extension-api-server/)
-   and use the [aggregator](/docs/tasks/extend-kubernetes/configure-aggregation-layer/)
-   to make it seamless for clients.
+## API groups and versioning
 
-## Enabling or disabling API groups
+To make it easier to eliminate fields or restructure resource representations,
+Kubernetes supports multiple API versions, each at a different API path, such
+as `/api/v1` or `/apis/rbac.authorization.k8s.io/v1alpha1`.
 
-Certain resources and API groups are enabled by default. They can be enabled or disabled by setting `--runtime-config`
-as a command line option to the kube-apiserver.
+Versioning is done at the API level rather than at the resource or field level
+to ensure that the API presents a clear, consistent view of system resources
+and behavior, and to enable controlling access to end-of-life and/or
+experimental APIs.
 
-`--runtime-config` accepts comma separated values. For example: to disable batch/v1, set
-`--runtime-config=batch/v1=false`; to enable batch/v2alpha1, set `--runtime-config=batch/v2alpha1`.
-The flag accepts comma separated set of key=value pairs describing runtime configuration of the API server.
+Refer to [API versions reference](/docs/reference/using-api/api-overview/#api-versioning)
+for more details on the API version level definitions.
 
-{{< note >}}Enabling or disabling groups or resources requires restarting the kube-apiserver and the
-kube-controller-manager to pick up the `--runtime-config` changes.{{< /note >}}
+To make it easier to evolve and to extend its API, Kubernetes implements
+[API groups](/docs/reference/using-api/api-overview/#api-groups) that can be
+[enabled or disabled](/docs/reference/using-api/api-overview/#enabling-or-disabling).
 
-## Persistence
+## API Extension
 
-Kubernetes stores its serialized state in terms of the API resources by writing them into
-{{< glossary_tooltip term_id="etcd" >}}.
+The Kubernetes API can be extended in one of two ways:
 
+1. [Custom resources](/docs/concepts/extend-kubernetes/api-extension/custom-resources/)
+   let you declaratively define how the API server should provide your chosen resource API.
+1. You can also extend the Kubernetes API by implementing an
+   [aggregation layer](/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/).
 
 ## {{% heading "whatsnext" %}}
 
-[Controlling API Access](/docs/reference/access-authn-authz/controlling-access/) describes
-how the cluster manages authentication and authorization for API access.
-
-Overall API conventions are described in the
-[API conventions](https://git.k8s.io/community/contributors/devel/sig-architecture/api-conventions.md#api-conventions)
-document.
-
-API endpoints, resource types and samples are described in the [API Reference](/docs/reference/kubernetes-api/).
+- Learn how to extend the Kubernetes API by adding your own
+  [CustomResourceDefinition](/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/).
+- [Controlling API Access](/docs/reference/access-authn-authz/controlling-access/) describes
+  how the cluster manages authentication and authorization for API access.
+- Learn about API endpoints, resource types and samples by reading
+  [API Reference](/docs/reference/kubernetes-api/).