Merge branch 'update-index-files-administration-1' into 'master'

Docs: Rename index.md to _index.md in administration docs - 1

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/180659



Merged-by: Evan Read <eread@gitlab.com>
Approved-by: Evan Read <eread@gitlab.com>
Co-authored-by: Marcel Amirault <mamirault@gitlab.com>