Document the new Cluster Management Project Template

What does this MR do?

This MR add docs about the Cluster Management Project Template. The idea is to:

• Explain how it works
• How can users from GMA v1 and v2 migrate into it
• Add a sidebar navigation right below cluster management project
• Update links that fostered the use of GMA v2 to use the proj template instead.
• The GMA page should become a simpler page with just a brief note about v2 deprecation and v1 removal, then just point with a link to the old docs from the 13.12 version. But this last topic can be done in a separate MR.

Related issues

#327908 (closed)

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