Add API documentation for pending members

What does this MR do and why?

We recently added two API endpoints for managing/approving pending members when a group hits it's user cap.

This MR adds the corresponding documentation

Screenshots or screen recordings

Link for convenience: https://gitlab.com/gitlab-org/gitlab/-/blob/08720b09c8956eb59277b8ef5bc0fc104c09b987/doc/api/members.md

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.