Docs: Cluster Management Project Template with the Agent

Problem to solve

• For gitlab-org/configure&8 (comment 711341115)
• Adapt Cluster Management Project Template to use the Agent

Further details

• Adjust the template itself to use the Agent (feature) => gitlab-org/project-templates/cluster-management!24 (merged)
• Update docs => !72879 (merged)

Who can address the issue

~"group::configure"