Merge branch 'docs/refactor-cluster' into 'master'

Refactor  Cluster docs

Closes #39952

See merge request gitlab-org/gitlab-ce!15418

(cherry picked from commit d41e66cb)

644a2941 Add info about the installed apps in the Cluster docs
b7a01e08 Add info on creating your own k8s cluster from the cluster page
ee7ecc81 Add Ingress description from official docs
3b9773fc Change to Google Kubernetes Cluster and add internal links
bacf671a Move installation of apps higher

doc/user/project/clusters/index.md

-# Connecting GitLab with GKE
+# Connecting GitLab with a Kubernetes cluster
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/35954) in 10.1.
 
 CAUTION: **Warning:**
 The Cluster integration is currently in **Beta**.
 
-Connect your project to Google Container Engine (GKE) in a few steps.
-
 With a cluster associated to your project, you can use Review Apps, deploy your
-applications, run your pipelines, and much more in an easy way.
+applications, run your pipelines, and much more, in an easy way.
+
+Connect your project to Google Kubernetes Engine (GKE) or your own Kubernetes
+cluster in a few steps.
 
 NOTE: **Note:**
 The Cluster integration will eventually supersede the
@@ -30,36 +31,58 @@ prerequisites must be met:
 - You must have Master [permissions] in order to be able to access the **Cluster**
   page.
 
-If all of the above requirements are met, you can proceed to add a new cluster.
+If all of the above requirements are met, you can proceed to add a new GKE
+cluster.
 
 ## Adding a cluster
 
 NOTE: **Note:**
 You need Master [permissions] and above to add a cluster.
 
+There are two options when adding a new cluster; either use Google Kubernetes
+Engine (GKE) or provide the credentials to your own Kubernetes cluster.
+
 To add a new cluster:
 
-1. Navigate to your project's **CI/CD > Cluster** page.
-1. Connect your Google account if you haven't done already by clicking the
-   "Sign-in with Google" button.
-1. Fill in the requested values:
+1. Navigate to your project's **CI/CD > Cluster** page
+1. If you want to let GitLab create a cluster on GKE for you, go through the
+   following steps, otherwise skip to the next one.
+    1. Click on **Create with GKE**
+    1. Connect your Google account if you haven't done already by clicking the
+       **Sign in with Google** button
+    1. Fill in the requested values:
       - **Cluster name** (required) - The name you wish to give the cluster.
       - **GCP project ID** (required) - The ID of the project you created in your GCP
         console that will host the Kubernetes cluster. This must **not** be confused
         with the project name. Learn more about [Google Cloud Platform projects](https://cloud.google.com/resource-manager/docs/creating-managing-projects).
-  - **Zone** - The zone under which the cluster will be created. Read more about
-    [the available zones](https://cloud.google.com/compute/docs/regions-zones/).
+      - **Zone** - The [zone](https://cloud.google.com/compute/docs/regions-zones/)
+        under which the cluster will be created.
       - **Number of nodes** - The number of nodes you wish the cluster to have.
-  - **Machine type** - The machine type of the Virtual Machine instance that
-    the cluster will be based on. Read more about [the available machine types](https://cloud.google.com/compute/docs/machine-types).
+      - **Machine type** - The [machine type](https://cloud.google.com/compute/docs/machine-types)
+        of the Virtual Machine instance that the cluster will be based on.
       - **Project namespace** - The unique namespace for this project. By default you
         don't have to fill it in; by leaving it blank, GitLab will create one for you.
-1. Click the **Create cluster** button.
+1. If you want to use your own existing Kubernetes cluster, click on
+   **Add an existing cluster** and fill in the details as described in the
+   [Kubernetes integration](../integrations/kubernetes.md) documentation.
+1. Finally, click the **Create cluster** button
 
-After a few moments your cluster should be created. If something goes wrong,
+After a few moments, your cluster should be created. If something goes wrong,
 you will be notified.
 
-Now, you can proceed to [enable the Cluster integration](#enabling-or-disabling-the-cluster-integration).
+You can now proceed to install some pre-defined applications and then
+enable the Cluster integration.
+
+## Installing applications
+
+GitLab provides a one-click install for various applications which will be
+added directly to your configured cluster. Those applications are needed for
+[Review Apps](../../../ci/review_apps/index.md) and [deployments](../../../ci/environments.md).
+
+| Application | GitLab version | Description |
+| ----------- | :------------: | ----------- |
+| [Helm Tiller](https://docs.helm.sh/) | 10.2+ | Helm is a package manager for Kubernetes and is required to install all the other applications. It will be automatically installed as a dependency when you try to install a different app. It is installed in its own pod inside the cluster which can run the `helm` CLI in a safe environment. |
+| [Ingress](https://kubernetes.io/docs/concepts/services-networking/ingress/) | 10.2+ | Ingress can provide load balancing, SSL termination and name-based virtual hosting. It acts as a web proxy for your applications and is useful if you want to use [Auto DevOps](../../../topics/autodevops/index.md) or deploy your own web apps. |
 
 ## Enabling or disabling the Cluster integration
 
@@ -88,12 +111,3 @@ To remove the Cluster integration from your project, simply click on the
 and [add a cluster](#adding-a-cluster) again.
 
 [permissions]: ../../permissions.md
-
-## Installing applications
-
-GitLab provides a one-click install for
-[Helm Tiller](https://docs.helm.sh/) and
-[Ingress](https://kubernetes.io/docs/concepts/services-networking/ingress/)
-which will be added directly to your configured cluster.
-
-![Cluster application settings](img/cluster-applications.png)