chore: Update devices documentation with design details (#633)

* chore: Update devices documentation with design details

This documents the current code base as is with a higher level overview. This may deserve a better home after we've removed the original API, but seems Ok fr now.

* chore: fix lint errors in readme

* chore: update device traits by protocol

* chore: Document combined mqtt channels

* chore: Fix lint errors

* chore: Add details about test structure

* chore: fix typo

Author: allenporter