Add documentation on Custom Models testing

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/168189



Merged-by: Jon Glassman <jglassman@gitlab.com>
Approved-by: Jon Glassman <jglassman@gitlab.com>
Reviewed-by: Igor Drozdov <idrozdov@gitlab.com>
Co-authored-by: Manoj M J <mmj@gitlab.com>