Merge branch 'migrate-aigw-setup-docs' into 'master'

Migrate AI Gateway documentation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/168628



Merged-by: Jon Glassman <jglassman@gitlab.com>
Approved-by: Manoj M J <mmj@gitlab.com>
Approved-by: Jon Glassman <jglassman@gitlab.com>
Reviewed-by: Manoj M J <mmj@gitlab.com>
Co-authored-by: Mohamed Hamda <mhamda@gitlab.com>