WIP: Revamp of CI job docs: rules, only, except

What does this MR do?

This is a guide for using the rules CI keyword, along with differences to only/except, and some tips on migrating from only/except to rules.

I adopted the strategy of writing out everything I know about the topic, a bit as if no other documentation exists for this. Then, when finished, I'll compare to the old documentation, remove duplication, crosslink for SSoT, and try to remove as much as possible from the YAML reference to keep it as simple/dry as possible.

Still a very rough WIP while working out ideas.

Potential things to include:

• More migration tips?
  • What needs to be added/removed from the rules to only/except "translation" table?
• Move majority of only / except documentation out of the main YAML reference to the end of this new doc, and crosslink?
• Do we need more discussion about pipelines themselves, or is crosslinking enough? Is the pipelines page missing any details needed to make full use of rules? Should we have a clear list of "types" of pipelines that can be triggered, to be aware that "jobs can be added to all these pipelines"
• What are the key pain points, and how can we address them in this doc without calling them pain points?
• How to deal with duplicate pipelines trouble. Explain it here in detail, and link from other pages for SSoT?
• How to bring in workflow: rules?
  • Engineering question: What happens when you mix workflow: rules with job-defined rules?
• Need to add more crosslinking.
• Finalize section headers

Related issues

Related to #217446 (closed)

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