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

Document GitLab image registry and CI/CD job token based authentication

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183865



Merged-by: Brendan Lynch <blynch@gitlab.com>
Approved-by: Tomasz Maczukin <tomasz@maczukin.pl>
Reviewed-by: Tomasz Maczukin <tomasz@maczukin.pl>
Reviewed-by: Brendan Lynch <blynch@gitlab.com>
Co-authored-by: Roshni Sarangadharan <rsarangadharan@gitlab.com>