Merge branch 'andysoiron/add-subgroups-to-group-webhook-documentation' into 'master'

Add subgroups to group webhook documentation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/129777



Merged-by: Ashraf Khamis <akhamis@gitlab.com>
Approved-by: Ashraf Khamis <akhamis@gitlab.com>
Co-authored-by: Andy Soiron <asoiron@gitlab.com>