Begin splitting merge request approvals page

https://docs.gitlab.com/ee/user/project/merge_requests/merge_request_approvals.html is a beast that has grown out of control, and needs to be split apart as part of the CTRT (concept + task + resources + troubleshooting).

This MR will not accomplish the CTRT rewrite, because it will make this merge request unreviewable. It lays groundwork:

1. Moves file to become the index file of a new subdirectory:
   • Old filename: doc/user/project/merge_requests/merge_request_approvals.md
   • New filename: doc/user/project/merge_requests/approvals/index.md
2. Preserves old Disqus comments
3. Splits off portions of the content into two new sibling pages, rules.md and settings.md
4. Updates outbound crosslinks from these pages
5. Updates inbound crosslinks TO these pages
6. TODO: update in-app links to the content (hold until the end of this MR because as soon as I edit backend files, the pipeline length increases dramatically)
7. TODO SEPARATELY: update left nav gitlab-docs!1749 (merged)
8. TODO SEPARATELY: better cross-linking between this new page approvals/index.md and doc/user/admin_area/merge_requests_approvals.md which I bet users confuse all the time
9. TODO SEPARATELY: update www-gitlab-com gitlab-com/www-gitlab-com!80881 (merged)

Related issue

Related to #324948 (closed)