gitlab-ci: shorten job prefixes in 3.22 release branch
This helps to maximize the amount of information visible in the GitLab web interface.
Also document their meaning in the developer documentation and in the CI configuration file directly.
This backports !6744 (merged) to the release
branch, and adds a maintainer guide documentation update missed in that MR.
See: https://gitlab.com/gitlab-org/gitlab/-/issues/8496
Backport: release:HEAD^2