Merge branch 'prep-service-desk-doc-improvements' into 'master'

Move Service Desk doc to own folder

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/127314



Merged-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: Kevin Chu <kchu@gitlab.com>