Refactor merge requests documentation

The current documentation on merge requests is split in many places, so the idea is to move all documents under a single location following the new doc restructure.

We currently have:

• http://docs.gitlab.com/ce/workflow/merge_requests.html The main merge requests docs.
• http://docs.gitlab.com/ce/gitlab-basics/add-merge-request.html Very old docs, should be merged with the one above and expand later under university.gitlab.com
• http://docs.gitlab.com/ce/workflow/revert_changes.html
• http://docs.gitlab.com/ce/workflow/cherry_pick_changes.html
• http://docs.gitlab.com/ce/workflow/wip_merge_requests.html
• http://docs.gitlab.com/ce/workflow/merge_when_build_succeeds.html
• http://docs.gitlab.com/ce/workflow/authorization_for_merge_requests.html

Also in EE:

• http://docs.gitlab.com/ee/workflow/merge_request_approvals.html
• http://docs.gitlab.com/ee/workflow/ff_merge.html
• http://docs.gitlab.com/ee/workflow/rebase_before_merge.html

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Steps

• The main doc will be user/project/merge_requests.md and it will have links to all other docs (move from workflow/merge_requests.md)
• Move "Only allow merge requests to be merged if the build succeeds" from main doc to user/project/merge_requests/merge_when_build_succeeds.md
• Move "Merge Requests versions" to user/project/merge_requests/versions.md
• Move "Ignore whitespace changes in Merge Request diff view" to a its own doc or a new one, maybe one with MR tips. (leaving as is for now)
• workflow/revert_changes.md to user/project/merge_requests/revert_changes.md
• workflow/cherry_pick_changes.md to user/project/merge_requests/cherry_pick_changes.md
• workflow/wip_merge_requests.md to user/project/merge_requests/work_in_progress_merge_requests.md
• workflow/merge_when_build_succeeds.md to user/project/merge_requests/merge_when_build_succeeds.md
• workflow/authorization_for_merge_requests.md to user/project/merge_requests/authorization_for_merge_requests.md
• Move gitlab-basics/add-merge-request.mdto user/project/merge_requests/create_a_merge_request.md (leaving as is for now)

Doc references:

app/views/projects/_merge_request_settings.html.haml:14:            = link_to icon('question-circle'), help_page_path('workflow/merge_requests', anchor: 'only-allow-merge-requests-to-be-merged-if-the-build-succeeds')
app/views/projects/merge_requests/show/_how_to_merge.html.haml:51:          %a{href: 'https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/workflow/merge_requests.md#checkout-merge-requests-locally', target: '_blank'} following these guidelines

doc/api/README.md:25:- [Merge Requests](merge_requests.md)
doc/workflow/forking_workflow.md:49:create the [merge request](merge_requests.md).
doc/workflow/README.md:3:- [Authorization for merge requests](authorization_for_merge_requests.md)
doc/workflow/README.md:24:- [Merge Requests](merge_requests.md)
doc/workflow/README.md:27:- ["Work In Progress" Merge Requests](wip_merge_requests.md)

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Needed for linking in gitlab-com/www-gitlab-com!3048 (merged) cc @marcia