[llvm] [docs] Add guide for Undefined Behavior (PR #119220)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

Yes, I agree that we can add a few more things. I'm sure we'll get questions about it, so we can refine as we go.
I just wanted to get something started 🙂  And something to like from the automatic code reviewer. We need more of these automatic things to improve the quality of the code IMHO.

https://github.com/llvm/llvm-project/pull/119220