Fix new SAML Group Links API

GitLab 15.3 will introduce REST API endpoints for SAML Group Links as implemented in !93177 (merged) 🎉 .

During the review of multiple Pull Requests on go-gitlab and the GitLab Terraform Provider I've noticed that the current implementation was lacking documentation, a get endpoint to get a single SAML Group link and that the response of the create endpoint was unintentionally always empty.

I've corrected these things with this MR and truly hope it can still make it into GitLab 15.3 to not release it as-is - Especially the bug in the create endpoint would mean weird hacks for clients which already want to implement it 🤝

/cc @nmezzopera do you mind helping me to triage this one efficiently so that it hopefully can make it into 15.3?