Validating doc syncing (likely) works in PR CI

[huonw]

We've had a few changes recently that have broken the doc syncing CI, both of which are fixed in #56. It'd be good to have some sort of validation on PRs of the syncing process, to increase our assurance in it.

[thejcannon]

I'm +1, naturally, but do want to comment if anything breaks, we can easily fix and re-trigger. So it wouldn't be fatal.

Therefore I wouldn't want to spend more time on this (knowing how finicky testing GHA can be) than the savings would yield in time spent patching-and-re-running.

[huonw]

I broadly agree but I think it's worth a bit more attention that just a pure "time to fix" vs "time to improve" calculation, because I imagine it'll be easy for people to not notice that the process failed, and so we could easily go a few (dev) releases accidentally not updating the docs, leading to potential user confusion.

In any case, I think we could have more assurance without having to go all the way to running workflows in act, e.g.:

1. refactor the workflow to call an in-repo action or script that runs pants and updates the files
2. have PR CI also call that action/script for a fixed version (e.g. 2.17.1 or something) and check it succeeds without making any file changes (this would've caught both problems fixed in Fix doc syncing for .x removal #56).

In any case, this issue is just to record a potential improvement, it's doesn't block the switch-over.