Merge branch 'eread/split-off-project-forks-api-documentation' into 'master'

Split off project forks API documentation

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167861



Merged-by: Marcel Amirault <mamirault@gitlab.com>
Approved-by: Marcel Amirault <mamirault@gitlab.com>
Co-authored-by: Evan Read <eread@gitlab.com>