Add developer documentation on our CI environment variables

What does this MR do?

Adds some basic documentation on our CI variables. This first iteration is specific to just ensuring there was an entry for each variable we have in the dev.gitlab.org environment. There is still a lot to be desired, like outlining which jobs these are used in, and what libraries in the code use them. But that seemed like just too much to tackle at the moment.

This at least makes them known, so they can be searched for.

Also missing from this list are any variable we have support for, but haven't overridden in our environments. That's probably the very next iteration to get in at some point.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened