[globalisel][docs] Add KnownBits Analysis documentation

Summary:
This is largely based off of the slides from the keynote

Depends on D69545

Reviewers: volkan, rovka, arsenm

Subscribers: wdng, arphaman, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69644