Resolve "Community contribution pipelines fail"

Why is this change being made?

Recent refactors to the CI config file (.gitlab-ci.yml to switch to incremental and combined build-and-deploy jobs broke the functionality of community-contributed merge requests from forks of the original repo.

This MR fixes those problems, as well as ensuring that all the master and MR builds on the original repo still work correctly.

Also (and importantly), it ensures that there are no jobs with rules having the "duplicate pipelines bug"

There is also some associated cleanup, documentation, and refactoring of the CI config file.

Closes #8863 (closed)