Document and tidy up CI

[pfmoore]

Let's capture all of this in an open issue though, instead of a closed PR.

Originally posted by @pradyunsg in #9826 (comment)

Things that should probably be documented:

1. How the code that skips tests when a PR doesn't touch certain files works.
2. What our policy is on using 3rd party github actions.
3. The New-RAMDisk.ps1 script - assuming it was copied from somewhere else on the web, let's add a link. If someone wrote it from scratch, let's document what it's doing and why!

Some of these can be done as comments in the code, but I'd suggest we have a section on the CI in the internal docs, even if it's only brief at the moment, it will act as a place to collect information over time.

Also one item to improve:

• Consolidate the check on when to run given tests into the determine-changes action, so we don't need to check if we're in a PR all over the place.

[pradyunsg]

If someone wrote it from scratch, let's document what it's doing and why!

#7263