Merge branch 'lower-case-index-doc' into 'master'

Add guideline to only allow lower case index names

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/107513



Merged-by: Amy Qualls <aqualls@gitlab.com>
Approved-by: Mayra Cabrera <mcabrera@gitlab.com>
Approved-by: Alex Ives <aives@gitlab.com>
Approved-by: Adam Hegyi <ahegyi@gitlab.com>
Approved-by: Alper Akgun <aakgun@gitlab.com>
Approved-by: Amy Qualls <aqualls@gitlab.com>
Co-authored-by: Steve Abrams <sabrams@gitlab.com>