Merge branch 'eread/combine-branch_related-content' into 'master'

Combine related Git documentation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157251



Merged-by: Suzanne Selhorn <sselhorn@gitlab.com>
Approved-by: Rudy Crespo <rcrespo@gitlab.com>
Co-authored-by: Evan Read <eread@gitlab.com>