Move legacy cluster APIs into OpenAPI namespace

What does this MR do and why?

Moves legacy cluster APIs into OpenAPI namespace, including:

• Adding/updating description and detail to match the markdown docs (project, group, instance)
• Grouping under the clusters tag
• Adding a success entry where missing

Screenshots or screen recordings

How to set up and validate locally

Refer to the description of one of the linked issues below.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/376859, https://gitlab.com/gitlab-org/gitlab/-/issues/376957, https://gitlab.com/gitlab-org/gitlab/-/issues/376915