Docs: admin index

What does this MR do?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/39688

Add an index for Admin docs:

• List all Admin-related settings, options, etc
• Remove the list on https://docs.gitlab.com/ce/README.html#administrator-documentation

Review Apps links:

• CE landing page: http://ce-docs-admin-index.178.62.207.141.xip.io/ce/
• CE Admin doc: http://ce-docs-admin-index.178.62.207.141.xip.io/ce/administration/