Move `workflow/merge_requests.md` to `user/project/merge_requests.md`

doc/user/project/merge_requests.md

doc/user/project/merge_requests/img/commit_compare.png

doc/user/project/merge_requests/img/merge_request_diff.png

...ge_requests/img/merge_request_diff_without_whitespace.png

...merge_requests/img/only_allow_merge_if_build_succeeds.png

doc/user/project/merge_requests/img/versions.png

doc/workflow/README.md

doc/workflow/merge_requests.md