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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Yeah so that's an annoying thing I've had to work around. Unlike RST the lines don't flow automatically regardless of the newlines. So I've tried to lay it out so that the paragraphs aren't too weird looking, but the text is still readable in an editor.

Also when you have a `[link](giant URL...............................goes here)` that renders into a single word `link`. So also tried to write
it so we'd end up with a full line there. Instead of:
```
Some text with a <link>.
Next sentence ends up here which looks strange given it's supposed to be part of the same paragraph
```

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