Update Kubernetes quickstart guide with instructions for v9.2 (elastic#3597)

This PR updates the [Quickstart: Monitor your Kubernetes cluster with
Elastic
Agent](https://www.elastic.co/docs/solutions/observability/get-started/quickstart-monitor-kubernetes-cluster-with-elastic-agent)
doc to include tje changes in the flow for the 9.2 version (using a
`helm` command instead of `kubectl` one) by adding a separate tab for
this version.

Disclaimer: we’re using simple tabs as applies switches do not support
version ranges yet.

Also reordered the tabs to start with the Serverless use case as we aim
to promote Serverless first.

Resolves elastic#2456

Author: vishaangelova

solutions/observability/get-started/quickstart-monitor-kubernetes-cluster-with-elastic-agent.md

@@ -15,13 +15,26 @@ products:
 
 In this quickstart guide, you’ll learn how to create the Kubernetes resources required to monitor your cluster infrastructure by using a single command to download, install, and configure {{agent}} in your Kubernetes cluster.
 
+:::{note}
+In {{stack}} versions 9.0 and 9.1, the quickstart uses [Kubectl](https://kubernetes.io/docs/reference/kubectl/) to install {{agent}} in a Kubernetes cluster. However, these versions also support using [Helm](https://helm.sh/docs/) charts, which is now the preferred method for installing {{agent}} on Kubernetes. If your cluster is on version 9.0 or 9.1, we recommend that you follow the [Install Elastic Agent on Kubernetes using Helm](/reference/fleet/install-on-kubernetes-using-helm.md) guide to deploy {{agent}}.
+:::
+
 ## Prerequisites [_prerequisites_2]
 
 :::::{tab-set}
 :group: stack-serverless
 
-::::{tab-item} {{stack}}
-:sync: stack
+::::{tab-item} {{serverless-short}}
+:sync: serverless
+
+* An {{obs-serverless}} project. To learn more, refer to [Create an Observability project](/solutions/observability/get-started.md).
+* A user with the **Admin** role or higher (required to onboard system logs and metrics). To learn more, refer to [Assign user roles and privileges](/deploy-manage/users-roles/cloud-organization/user-roles.md#general-assign-user-roles).
+* A running Kubernetes cluster with internet access
+* [Helm](https://helm.sh/docs/)
+::::
+
+::::{tab-item} {{stack}} 9.2 and later
+:sync: stack-9.2
 
 * A running {{stack}} deployment, either self-managed or orchestrated by platforms like {{ech}}, {{ece}}, or {{eck}}, with internet access. To get started quickly, try out [{{ecloud}}](https://cloud.elastic.co/registration?page=docs&placement=docs-body).
 * A user with the `superuser` [built-in role](elasticsearch://reference/elasticsearch/roles.md) or the privileges required to onboard data.
@@ -33,16 +46,23 @@ In this quickstart guide, you’ll learn how to create the Kubernetes resources
 :::
 
 * A running Kubernetes cluster with internet access
-* [Kubectl](https://kubernetes.io/docs/reference/kubectl/)
+* [Helm](https://helm.sh/docs/)
 ::::
 
-::::{tab-item} Serverless
-:sync: serverless
+::::{tab-item} {{stack}} 9.0-9.1
+:sync: stack-9.0-9.1
+
+* A running {{stack}} deployment, either self-managed or orchestrated by platforms like {{ech}}, {{ece}}, or {{eck}}, with internet access. To get started quickly, try out [{{ecloud}}](https://cloud.elastic.co/registration?page=docs&placement=docs-body).
+* A user with the `superuser` [built-in role](elasticsearch://reference/elasticsearch/roles.md) or the privileges required to onboard data.
+
+:::{dropdown} Expand to view required privileges
+* [**Cluster**](elasticsearch://reference/elasticsearch/security-privileges.md#privileges-list-cluster): `['monitor', 'manage_own_api_key']`
+* [**Index**](elasticsearch://reference/elasticsearch/security-privileges.md#privileges-list-indices): `{ names: ['logs-*-*', 'metrics-*-*'], privileges: ['auto_configure', 'create_doc'] }`
+* [**Kibana**](/deploy-manage/users-roles/cluster-or-deployment-auth/kibana-privileges.md): `{ spaces: ['*'], feature: { fleet: ['all'], fleetv2: ['all'] } }`
+:::
 
-* An {{obs-serverless}} project. To learn more, refer to [Create an Observability project](/solutions/observability/get-started.md).
-* A user with the **Admin** role or higher (required to onboard system logs and metrics). To learn more, refer to [Assign user roles and privileges](/deploy-manage/users-roles/cloud-organization/user-roles.md#general-assign-user-roles).
 * A running Kubernetes cluster with internet access
-* [Helm](https://helm.sh/docs/)
+* [Kubectl](https://kubernetes.io/docs/reference/kubectl/)
 ::::
 
 :::::
@@ -61,15 +81,36 @@ The installation command provided by the UI during the quickstart cannot be used
 :::::{tab-set}
 :group: stack-serverless
 
-::::{tab-item} Elastic Stack
-:sync: stack
+::::{tab-item} {{serverless-short}}
+:sync: serverless
 
-1. In {{kib}}, go to the **Observability** UI and click **Add Data**.
+1. Go to your [{{obs-serverless}}](/solutions/observability/get-started.md) project, then go to **Add data**.
 2. In the **What do you want to monitor?** section, select **Kubernetes**, and then select **Elastic Agent: Logs & Metrics**.
-
 3. To install {{agent}} on your host, copy and run the install command.
 
-    By running this command, you use `kubectl kustomize` to download a manifest file, inject deployment-specific data such as the API key generated by {{kib}} for the acting user, and create the Kubernetes resources for {{agent}}.
+    By running this command, you use the Helm package manager to install and configure an instance of the {{agent}} Helm chart with additional deployment-specific data such as the API key generated by {{kib}} for the acting user.
+
+    The Helm chart also includes a default installation of [`kube-state-metrics` (KSM)](https://github.com/kubernetes/kube-state-metrics), which is required by the Kubernetes integration to collect cluster-level metrics.
+
+    If you encounter an error during the installation, refer to [Troubleshooting](#monitor-k8s-with-agent-troubleshooting).
+
+    :::{dropdown} Details about the install command
+    The install command provided by the UI may be similar to:
+
+    ```sh subs=true
+    helm repo add elastic https://helm.elastic.co/ && helm install elastic-agent elastic/elastic-agent --version {{version.stack}} -n kube-system --set outputs.default.url=https:<elasticsearch-url>:443 --set kubernetes.onboardingID=<internal-id> --set kubernetes.enabled=true --set outputs.default.type=ESPlainAuthAPI --set outputs.default.api_key=$(echo "<api-key>" | base64 -d)
+    ```
+
+    Where:
+
+    - `elastic-agent` is the name of the specific installation of the Helm chart, known as **release name**.
+    - `elastic/elastic-agent` defines the name of the chart to install, using the format `<repository>/<chart-name>`.
+    - {{version.stack}} is the version of the {{agent}} Helm chart to be installed.
+    - `kube-system` is the namespace where {{agent}} is to be installed.
+    - `--set` parameters add configuration values specific to the serverless project, the acting user, and the deployment method of the Helm chart.
+
+       Refer to [Install standalone Elastic Agent on Kubernetes using Helm](/reference/fleet/example-kubernetes-standalone-agent-helm.md#agent-standalone-helm-example-install) for a more detailed explanation of the configuration options used.
+    :::
 
 4. Go back to the **Kubernetes: Logs & Metrics** page in {{kib}}.
 
@@ -79,10 +120,10 @@ The installation command provided by the UI during the quickstart cannot be used
 
 ::::
 
-::::{tab-item} Serverless
-:sync: serverless
+::::{tab-item} {{stack}} 9.2 and later
+:sync: stack-9.2
 
-1. Go to your [{{obs-serverless}}](/solutions/observability/get-started.md) project, then go to **Add Data**.
+1. In {{kib}}, go to the **Observability** overview page, and click **Add Data**.
 2. In the **What do you want to monitor?** section, select **Kubernetes**, and then select **Elastic Agent: Logs & Metrics**.
 3. To install {{agent}} on your host, copy and run the install command.
 
@@ -118,6 +159,24 @@ The installation command provided by the UI during the quickstart cannot be used
 
 ::::
 
+::::{tab-item} {{stack}} 9.0-9.1
+:sync: stack-9.0-9.1
+
+1. In {{kib}}, go to the **Observability** UI and click **Add Data**.
+2. In the **What do you want to monitor?** section, select **Kubernetes**, and then select **Elastic Agent: Logs & Metrics**.
+
+3. To install {{agent}} on your host, copy and run the install command.
+
+    By running this command, you use `kubectl kustomize` to download a manifest file, inject deployment-specific data such as the API key generated by {{kib}} for the acting user, and create the Kubernetes resources for {{agent}}.
+
+4. Go back to the **Kubernetes: Logs & Metrics** page in {{kib}}.
+
+    There might be a slight delay before data is ingested. When ready, you will see the message **We are monitoring your cluster**.
+
+5. Click **Explore Kubernetes cluster** to navigate to dashboards and explore your data.
+
+::::
+
 :::::
 
 ## Visualize your data [_visualize_your_data_2]
@@ -138,17 +197,25 @@ Refer to [Observability overview](/solutions/observability/get-started/what-is-e
 ::::{tab-set}
 :group: stack-serverless
 
-:::{tab-item} Elastic Stack
-:sync: stack
+:::{tab-item} {{serverless-short}}
+:sync: serverless
 
-To uninstall {{agent}} and the Kubernetes resources installed with `kubectl`:
+To uninstall {{agent}} and the Kubernetes resources installed with Helm, run:
 
-1. Copy the `kubectl` quickstart command for installing {{agent}} described in the [Collect your data](#_collect_your_data_2) section.
-2. Replace `| kubectl apply -f-` with `| kubectl delete -f-`, then run the command.
+```sh
+helm uninstall <release-name> -n <namespace> <1>
+```
+1. Substitute `<release-name>` with the release name and `<namespace>` with the namespace used in the quickstart command described in the [Collect your data](#_collect_your_data_2) section.
+
+If you used the default values from the quickstart, the command would be:
+
+```sh
+helm uninstall elastic-agent -n kube-system
+```
 :::
 
-:::{tab-item} Serverless
-:sync: serverless
+:::{tab-item} {{stack}} 9.2 and later
+:sync: stack-9.2
 
 To uninstall {{agent}} and the Kubernetes resources installed with Helm, run:
 
@@ -164,6 +231,15 @@ helm uninstall elastic-agent -n kube-system
 ```
 :::
 
+:::{tab-item} {{stack}} 9.0-9.1
+:sync: stack-9.0-9.1
+
+To uninstall {{agent}} and the Kubernetes resources installed with `kubectl`:
+
+1. Copy the `kubectl` quickstart command for installing {{agent}} described in the [Collect your data](#_collect_your_data_2) section.
+2. Replace `| kubectl apply -f-` with `| kubectl delete -f-`, then run the command.
+:::
+
 ::::
 
 ## Troubleshooting [monitor-k8s-with-agent-troubleshooting]