Explain generate GraphQL schema and how to edit

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/149461



Merged-by: Peter Leitzen <pleitzen@gitlab.com>
Approved-by: Amy Qualls <aqualls@gitlab.com>
Approved-by: Kati Paizee <kpaizee@gitlab.com>
Approved-by: Peter Leitzen <pleitzen@gitlab.com>
Reviewed-by: Kati Paizee <kpaizee@gitlab.com>
Co-authored-by: Kati Paizee <kpaizee@gitlab.com>
Co-authored-by: Amy Qualls <aqualls@gitlab.com>
Co-authored-by: Christopher Guitarte <cguitarte@gitlab.com>