New doc: how to install GitLab on Azure
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
- Adds new doc: install GitLab on Azure, follow up from community contribution (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13041)
- Indexes other install guides from https://docs.gitlab.com/ce/install/