Refactor merge requests documentation
## What does this MR do?
Add more information on merge requests.
## Moving docs to a new location?
See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location
- [x] Make sure the old link is not removed and has its contents replaced with a link to the new location.
- [x] Make sure internal links pointing to the document in question are not broken.
- [x] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
- [x] If working on CE, submit an MR to EE with the changes as well.
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Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/21857
See merge request !6202
7d2bd6b91b
