Move merge request versions to its own document

doc/user/project/merge_requests/versions.md

+# Merge requests versions
+
+> Will be [introduced][ce-5467] in GitLab 8.12.
+
+Every time you push to a branch that is tied to a merge request, a new version
+of merge request diff is created. When you visit a merge request that contains
+more than one pushes, you can select and compare the versions of those merge
+request diffs.
+
+By default, the latest version of changes is shown. However, you
+can select an older one from version dropdown.
+
+![Merge Request Versions](img/versions.png)
+
+---
+
+>**Note:**
+Merge request versions are based on push not on commit. So, if you pushed 5
+commits in a single push, it will be a single option in the dropdown. If you
+pushed 5 times, that will count for 5 options.
+
+[ce-5467]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5467