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