Add documentation for `approval_settings` endpoint
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=235514) - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=235514) </details> <!--IssueSummary end--> We seem to be missing documentation on https://docs.gitlab.com/ee/api/merge_request_approvals.html for the `approval_settings` endpoint, e.g. `/api/v4/projects/8/merge_requests/2/approval_settings`. I noticed this as part of https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39237 and https://gitlab.com/gitlab-org/gitlab/-/issues/10294. This should be fairly simple to add as the fields are largely similar to other endpoints on the page already. /cc @m_gill
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