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/
Edited by Marcia Ramos