Merge branch 'rd-doc-mr-approval-requirements' into 'master'

Document MR approval policy requirements

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163417



Merged-by: Lysanne Pinto <lpinto@gitlab.com>
Approved-by: Martin Čavoj <mcavoj@gitlab.com>
Approved-by: Ryan Lehmann <rlehmann@gitlab.com>
Approved-by: Lysanne Pinto <lpinto@gitlab.com>
Reviewed-by: Martin Čavoj <mcavoj@gitlab.com>
Reviewed-by: Ryan Lehmann <rlehmann@gitlab.com>
Co-authored-by: Russell Dickenson <rdickenson@gitlab.com>