Cleanup and consolidate the CI yaml docs
What does this MR do?
I started adding some missing info in the CI yaml docs, but I git carried away and cleaned it up :)
Are there points in the code the reviewer needs to double check?
Some links where changed, the lint passes, but there might some a possibility I forgot to change the anchor links some place :)
Once this is approved I'll provide an EE MR, since there will probably be conflicts.
Why was this MR needed?
General cleanup.
What are the relevant issue numbers?
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/42914, https://gitlab.com/gitlab-org/gitlab-ce/issues/18945