* 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