WIP: Adds documentation for "Support Kubernetes RBAC for GitLab Managed Apps"

What does this MR do?

Adds documentation for enabling Kubernetes RBAC for GitLab Managed Apps for adding existing clusters and when creating new GKE clusters

What are the relevant issue numbers?

Closes #29398 (closed)

To do

• Write documentation about RBAC support for GitLab Managed Apps (behind a feature flag)
  • Enable feature flag
  • What are the limitations
  • What are the permissions involved
  • What is supported
• Link More information to these docs

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides