Docs: add admin index
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
- Closes #4222 (closed)
- Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15724 to EE
- Adds EE-only items from https://docs.gitlab.com/ee/README.html#administrator-documentation
Review Apps:
- EE landing page: http://ee-docs-admin-index-ee.178.62.207.141.xip.io/ee
- EE admin doc: http://ee-docs-admin-index-ee.178.62.207.141.xip.io/ee/administration/
cc/ @axil
Edited by 🤖 GitLab Bot 🤖