Merge branch 'meta/add-doc-mr-template' into 'master'

Add MR Documentation description template

Add an MR Documentation template for `gitlab-ce`.

See merge request !6145

.gitlab/merge_request_templates/Documentation.md

+See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html.
+
+## What does this MR do?
+
+(briefly describe what this MR is about)
+
+## Moving docs to a new location?
+
+See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location
+
+- [ ] Make sure the old link is not removed and has its contents replaced with a link to the new location.
+- [ ] Make sure internal links pointing to the document in question are not broken.
+- [ ] Search and replace any links referring to old docs in GitLab Rails app, specifically under the `app/views/` directory.
+- [ ] If working on CE, submit an MR to EE with the changes as well.