[globalisel][docs] Rewrite the pipeline overview

Summary:
Rewrite the pipeline overview to be more focused on the structure and
flexibility as well as highlight the increased usefulness of
MachineVerifier and increased testability resulting from the smaller
incremental passes approach.

The diagrams are lifted from the slides for the LLVMDev 2019 talk
'Generating Optimized Code with GlobalISel' and adapted to be readable on
the white background used in the docs.

Reviewers: volkan

Subscribers: rovka, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69456