Add an overview of Validity subsystems (#1161)

* migrate subsystem expansion PR from w3f/research

As the file at this new location included changes not present in
https://github.com/w3f/research/pull/87, this is effectively a
rebase, applied manually. I believe that I have successfully retained
all of, and only, the intended changes.

* add section on collators

* note why the overseer is the terminal message point for the validation subsystem

* add detail about how the validitiy system gets PoV blocks

* rename to Validity Subsystems Overview

* get rid of changes to the implementor's guide

I think it makes most sense to track and review the subsystems
overview in a different PR than the one which adds content to
the implementor's guide.

* punt misbehavior arbitration details to the future

* empty commit to rerun CI