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

[David Spickett via llvm-commits]

DavidSpickett wrote:

> Maybe that's getting a bit lengthy though...

Given we have so many "but this", "sometimes that" I'm wondering if it's not better for me to update one of the documentation pages, then link to that. Then we have more room to include them. Not sure which yet though.

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