Add initial doc for cluster management project

Mark as alpha and do not link yet as we still have to implement the
front-end.

doc/user/clusters/management_project.md

+# Cluster management project (alpha)
+
+CAUTION: **Warning:**
+This is an _alpha_ feature, and it is subject to change at any time without
+prior notice.
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/merge_requests/17866) in GitLab 12.4
+
+A project can be designated as the management project for a cluster.
+A management project can be used to run deployment jobs with
+Kubernetes
+[`cluster-admin`](https://kubernetes.io/docs/reference/access-authn-authz/rbac/#user-facing-roles)
+privileges.
+
+This can be useful for:
+
+- Creating pipelines to install cluster-wide applications into your cluster.
+- Any jobs that require `cluster-admin` privileges.
+
+## Permissions
+
+Only the management project will receive `cluster-admin` privileges. All
+other projects will continue to receive [namespace scoped `edit` level privileges](../project/clusters/index.md#rbac-cluster-resources).
+
+## Usage
+
+### Selecting a cluster management project
+
+This will be implemented as part of [this
+issue](https://gitlab.com/gitlab-org/gitlab/issues/32810).
+
+### Configuring your pipeline
+
+After designating a project as the management project for the cluster,
+write a [`.gitlab-ci,yml`](../../ci/yaml/README.md) in that project. For example:
+
+```yaml
+configure cluster:
+  stage: deploy
+  script: kubectl get namespaces
+  environment:
+    name: production
+```
+
+### Setting the environment scope **(PREMIUM)**
+
+[Environment
+scopes](../project/clusters/index.md#setting-the-environment-scope-premium)
+are usable when associating multiple clusters to the same management
+project.
+
+Each scope can only be used by a single cluster for a management project.
+
+For example, let's say the following Kubernetes clusters are associated
+to a management project:
+
+| Cluster     | Environment scope |
+| ----------- | ----------------- |
+| Development | `*`               |
+| Staging     | `staging`         |
+| Production  | `production`      |
+
+The the following environments set in
+[`.gitlab-ci.yml`](../../ci/yaml/README.md) will deploy to the
+Development, Staging, and Production cluster respectively.
+
+```yaml
+stages:
+- deploy
+
+configure development cluster:
+  stage: deploy
+  script: kubectl get namespaces
+  environment:
+    name: development
+
+configure staging cluster:
+  stage: deploy
+  script: kubectl get namespaces
+  environment:
+    name: staging
+
+configure production cluster:
+  stage: deploy
+  script: kubectl get namespaces
+  environment:
+    name: production
+```
+
+## Disabling this feature
+
+This feature is enabled by default. To disable this feature, disable the
+feature flag `:cluster_management_project`.
+
+To check if the feature flag is enabled on your GitLab instance,
+please ask an administrator to execute the following in a Rails console:
+
+```ruby
+Feature.enabled?(:cluster_management_project)     # Check if it's enabled or not.
+Feature.disable(:cluster_management_project)      # Disable the feature flag.
+```