Surface link to auth section index

What does this MR do?

While browsing for docs in the administration/auth area, I noticed there was no easy link from the administration index to the auth index.

This fixes that.

Ported to EE in: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9351.

Related issues

None.

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9351)

Review checklist

• Technical writer's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9351), CE-MR afterwards