Add customer-facing GraphQL authentication docs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/139613



Merged-by: Luke Duncalfe <lduncalfe@gitlab.com>
Approved-by: Lee Tickett <ltickett@gitlab.com>
Reviewed-by: Luke Duncalfe <lduncalfe@gitlab.com>
Reviewed-by: Lee Tickett <ltickett@gitlab.com>
Co-authored-by: Lee Tickett <ltickett@gitlab.com>
Co-authored-by: Luke Duncalfe <lduncalfe@eml.cc>