Add new subsection for merge methods

• Link to the merge request of the new page: gitlab!86309 (merged) pulled content from multiple pages to create https://docs.gitlab.com/ee/user/project/merge_requests/methods/

  • Child page: https://docs.gitlab.com/ee/user/project/merge_requests/fast_forward_merge.html becomes a redirect for now, but will time out in three months
  • Child page: https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.

(FYI to @trakos - thanks!)