Identify SSO documentation outside of docs and consolidate at least 90% into product docs

Also update: https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/123161320

1. SAML quick start @gerardo | content in doc is covered. Restructuring of doc page and possible deprecation of our quick start guide covered in gitlab-org/gitlab#390688 (closed)
2. Customer Success documentation @mmora | content in doc is covered. Nothing special to add to the existing official documentation.
3. SCIM questions and answers doc @alejguer