Merge branch 'docs/mg-explain-ci-job-clone-20231010' into 'master'

Explain basics of how a CI job clones the repo

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/133706



Merged-by: Marcel Amirault <mamirault@gitlab.com>
Approved-by: Marcel Amirault <mamirault@gitlab.com>
Co-authored-by: Manuel Grabowski <mgrabowski@gitlab.com>