Add an index page to admin area section

What does this MR do?

The Admin Area in the documentation has never had an index page.

This is the first step in documenting every single thing in the Admin Area. For now, it links out to relevant documentation where it currently exists.

CE version at: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25375.

Related issues

• Related to: https://gitlab.com/gitlab-org/gitlab-ce/issues/57874, https://gitlab.com/gitlab-com/gitlab-docs/issues/298, &548, gitlab-com&21 (closed), https://gitlab.com/gitlab-org/gitlab-ce/issues/55234

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25375

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25375)