Generate protocol docs via codegen

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

At present protocol documentation is very behind. We are not consistently generating it. Also, we keep making manual changes to document. We need to split it into two documents, the overview document which we maintain manually, and the list of versions, messages etc which is automatically generated. We should use mustache.