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

[David Spickett via llvm-commits]

DavidSpickett wrote:

I've updated the text with the feedback so far.

I think it could go on https://llvm.org/docs/GitHub.html instead as "what happens after merge" so going to look at that next.

https://github.com/llvm/llvm-project/pull/78292