Make sync_docs failures noisier by always posting a PR

[huonw]

This makes us more likely to notice problems in the automatic doc syncing like #284 earlier, by surfacing the failures as PRs, not just hidden in the actions UI.

The doc syncing usually happens automagically as part of the release process, in the background. If it fails, this is only visible in two ways:

• implicitly, in the lack of automation:sync-docs An automatic sync of documentation from the pantsbuild/pants repo -labelled PR
• by noticing the failure on the actions page: https://github.com/pantsbuild/pantsbuild.org/actions/workflows/sync_docs.yml

Both of these are easy to miss (very easy to miss that something that should happen didn't happen, and the actions page is only rarely visited!). There's no progress updates/reminder about the doc sync, nor is there a notification on failure.

In practice, we missed failures on the 2.24.x branch for a few releases (in #284).

This PR adds a "notification" about the failure, by posting the sync PR unconditionally. The release manager thus gets a notification (about their review request) and it's visible/easier for others to notice it too, in the list of PRs. When there's a failure, this PR includes some extra notes to highlight it.

Examples triggered from this branch:

• successful run (unchanged): Update docs site for version 2.22.1rc3 #291
• unsuccessful run (new): [FAILURE] Update docs site for version 2.24.0.dev3 #290

[thejcannon]

I like the idea!