Merge branch '429913-docs-page-project-group-sharing' into 'master'

Adds docs page for explaining project-group sharing

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143497



Merged-by: Ashraf Khamis <akhamis@gitlab.com>
Approved-by: Ashraf Khamis <akhamis@gitlab.com>
Reviewed-by: Lorena Ciutacu <lciutacu@gitlab.com>
Co-authored-by: Lorena Ciutacu <lciutacu@gitlab.com>
Co-authored-by: Christina Lohr <clohr@gitlab.com>