[docs] Add guide about Undefined Behavior (#119220)

Thanks Antonio Frighetto, John Regehr, and Nikita Popov for reviewing this!

Author: nunoplopes

llvm/docs/Reference.rst

@@ -52,6 +52,7 @@ LLVM and API reference documentation.
    TestingGuide
    TransformMetadata
    TypeMetadata
+   UndefinedBehavior
    XRay
    XRayExample
    XRayFDRFormat
@@ -120,6 +121,9 @@ LLVM IR
   Defines the LLVM intermediate representation and the assembly form of the
   different nodes.
 
+:doc:`Undefined Behavior (UB) <UndefinedBehavior>`
+  A guide on what UB/undef/poison are and when to use each one.
+
 :doc:`InAlloca`
   Description of the ``inalloca`` argument attribute.