Merge branch 'eread/rename-indexes-for-subscriptions-docs' into 'master'

Rename index.md to _index.md for subscriptions docs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/180633



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