[DOC] - Add docs for shared group mounting based on keycloak permissions

[aktech]

Preliminary Checks

• This issue is not a question, feature request, RFC, or anything other than a bug report. Please post those things in GitHub Discussions: https://github.com/nebari-dev/nebari/discussions

Summary

After this is merged: nebari-dev/nebari#2593 we would have the ability to optionally create shared directory for groups in keycloak, which have appropriate permissions. We need to add the relevant docs on how to achieve that.

Steps to Resolve this Issue

not a bug, just need to add documentation for a new feature.