Merge branch 'eread/rename-index-and-move-titles-to-frontmatter-for-rest-api-docs' into 'master'

Rename index and move titles to frontmatter for REST API docs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/180301



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