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

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

Thanks for the quick review! I've patched the things you mentioned.

I think having a section about APIs makes sense! For the and/or logic, do we have any API you want to mention? Or do you have an example off the top of your head that we can show?

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