Add reference for audit API group

This is a reference for audit.k8s.io/v1 API group generated from kubernetes-sigs/reference-docs/genref tool.
More specifically, it is generated using the following command:

```shell
./genref -include apiserver-audit
```

Author: tengqm

content/en/docs/reference/_index.md

@@ -52,7 +52,9 @@ client libraries:
 * [kubelet](/docs/reference/command-line-tools-reference/kubelet/) - The
   primary agent that runs on each node. The kubelet takes a set of PodSpecs
   and ensures that the described containers are running and healthy.
-* [kube-apiserver](/docs/reference/command-line-tools-reference/kube-apiserver/) - REST API that validates and configures data for API objects such as  pods, services, replication controllers.
+* [kube-apiserver](/docs/reference/command-line-tools-reference/kube-apiserver/) -
+  REST API that validates and configures data for API objects such as  pods,
+  services, replication controllers.
 * [kube-controller-manager](/docs/reference/command-line-tools-reference/kube-controller-manager/) - Daemon that embeds the core control loops shipped with Kubernetes.
 * [kube-proxy](/docs/reference/command-line-tools-reference/kube-proxy/) - Can
   do simple TCP/UDP stream forwarding or round-robin TCP/UDP forwarding across
@@ -64,13 +66,15 @@ client libraries:
 
 ## Config APIs
 
-* [kube-proxy configuration (v1alpha1)](/docs/reference/config-api/kube-proxy-config.v1alpha1/)
-* [kube-scheduler config (v1beta1)](/docs/reference/config-api/kube-scheduler-config.v1beta1/)
-
+This section hosts the documentation for "unpublished" APIs which are used to
+configure  kubernetes components or tools. Most of these APIs are not exposed
+by the API server in a RESTful way though they are essential for a user or an
+operator to use or manage a cluster.
 
-## Config APIs
-
-* [kubelet config (v1beta1)](/docs/reference/config-api/kubelet-config.v1beta1/)
+* [kubelet configuration (v1beta1)](/docs/reference/config-api/kubelet-config.v1beta1/)
+* [kube-scheduler configuration (v1beta1)](/docs/reference/config-api/kube-scheduler-config.v1beta1/)
+* [kube-proxy configuration (v1alpha1)](/docs/reference/config-api/kube-proxy-config.v1alpha1/)
+* [`audit.k8s.io/v1` API](/docs/reference/config-api/apiserver-audit.v1/)
 
 ## Design Docs