Add documentation for `approval_settings` endpoint
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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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