doc: Update gitaly team onboarding instructions
- Add reference to new google group https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/21356#note_1368985234
- Cc: @jcaigitlab on onboarding issues
cc @knayakgl
Issue https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/21356