Crosslink MR approvals API page

These two release post item MRs made me realize our merge request approvers user-facing documentation and API documentation were not cross-linked:

• Remove merge request approvers endpoint: gitlab-com/www-gitlab-com!79538 (merged)
• Remove project approvers endpoint: gitlab-com/www-gitlab-com!79539 (merged)

The real answer is a Resources section on the user-facing documentation page, but a full CTRT cleanup is out of scope for this small change.

Pages to crosslink

• https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html
• https://docs.gitlab.com/ee/api/merge_request_approvals.html

Related issues

Closes #327514 (closed)