Update workflow for PRs

[webbnh]

The current workflow is broken when triggered by a PR event (see arcalot/arcaflow-plugin-rtla#1): the GHA checkout uses a detached head from a temporary branch, and so the attempt to push the update fails. (When triggered by a branch update event, it runs on a "real branch", and there's no problem.)

This PR tweaks the update step of the workflow:

• after configuring Git but before making any changes, it switches to the the "head branch" from the PR instead of using the current checkout
• then it moves the updated contents into the README.md file and commits it to the branch
• then, after pushing the branch, it switches the checkout back to the original ref

These steps are taken only if the trigger was a pull request (and only if there is a difference in the generated documentation). The value of github.head_ref is only available for PRs, so none of this will work for other triggers. And, more importantly, I suspect that we don't want this step to run except for PRs -- that is, I don't think we want to bot making changes to branches when no one is supervising. (The current state disables this step if the branch is protected, but that doesn't seem strict enough to me.)

This PR also streamlines the difference check in a separate commit.

Note: until this PR is merged, you can try it out by modifying your plugin workflow to reference @PR-support instead of @main.

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[dbutenhof]

Hmm. That's ... interesting.

[webbnh]

I tested this using arcalot/arcaflow-plugin-rtla#2 and, once I got the wrinkles smoothed out, the docsgen run was able to update my branch and the workflow was able to reset the checkout to the original (which does not contain the update...but it was safe on my branch).

So, @dustinblack, you should be able to pop 5007ae2 and eb58c53 from your branch, and when you push the result up the bot should push the updates back on. (Assuming that your PR uses this PR's updated workflow.)

[webbnh]

I'll give @dustinblack a chance to look and comment before this is merged. (Dustin, feel free to do the merge if you are good with this change.)