Docs Only - Document cluster applications from our template

What does this MR do?

P.S.: This is a big MR, but it's a MAJOR copy/paste.

The changes related to this are all the files I created inside:

doc/user/infrastructure/clusters/manage/management_project_applications/*

It documents cluster applications from our Cluster Management Project Template

GitLab Managed apps CI/CD got deprecated in 14.0. The apps got moved to
a project template. Although, the documentation for installing those
apps still lives under the deprecated page for GMA CI/CD.

This change creates separate pages to document each supported app.
It also creates links from the project template documentation to each
documented app.

There's no completely new content being created here. This change basically consists of copy/paste of the documentation that already existed in https://docs.gitlab.com/ee/user/clusters/applications.html, but extracting each CI/CD app into its own page.

The only changes made are some replacements of:

- > [Introduced] ...
+ > [Introduced](https://gitlab.com/gitlab-org/project-templates/cluster-management/-/merge_requests/5) in GitLab 14.0.

to match the MR that introduces the Project Template.

I also replaced how applications are installed to reflect the template:

- The following configuration is required to install APP_XYZ using GitLab CI/CD:
-   APP_XYZ:
-   installed: true
+ Assuming you already have a [Cluster management project](../management_project.md) created from a
+ [management project template](../management_project_template.md), to install APP_XYZ you should
+ uncomment this line from your `helmfile.yaml`:
    path: applications/APP_XYZ/helmfile.yaml

and where the applications are configured:

- .gitlab/managed-apps/APP_XYZ/values.yaml
+ applications/APP_XYZ/values.yaml

Related issues

#333301 (closed)

Author's checklist

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