[GitHub][workflows] Add buildbot information comment to first merged PR from a...

[GitHub][workflows] Add buildbot information comment to first merged PR from a new contributor (#78292)

This change adds a comment to the first PR from a new contributor that
is merged, which tells them what to expect post merge from the build
bots.

How they will be notified, where to ask questions, that you're more
likely to be reverted than in other projects, etc. The information
overlaps with, and links to
https://llvm.org/docs/MyFirstTypoFix.html#myfirsttypofix-issues-after-landing-your-pr.
So that users who simply read the email are still aware, and know where
to follow up if they do get reports.

To do this, I have added a hidden HTML comment to the new contributor
greeting comment. This workflow will look for that to tell if the author
of the PR was a new contributor at the time they opened the merge. It
has to be done this way because as soon as the PR is merged, they are by
GitHub's definition no longer a new contributor and I suspect that their
author association will be "contributor" instead.

I cannot 100% confirm that without a whole lot of effort and probably
breaking GitHub's terms of service, but it's fairly cheap to work around
anyway. It seems rare / almost impossible to reopen a PR in llvm at
least, but in case it does happen the buildbot info comment has its own
hidden HTML comment. If we find this we will not post another copy of
the same information.