MCBP documentation framework

We need to add a framework for documenting the different commands we've got

Each opcode needs to be documented with the payload it takes, and its "availability" (public, private), stability (committed, evolving) and which privileges.

I haven't thought about the details of how to do this, but I think we should probably write this documentation in a format that allows us to generate:

1) struct / class / constant definitions
2) validator code
3) privilege check
4) descriptions for humans

In it's simplest form it could be as easy as we write stuff in markdown, and add a special "tag" in a formatted section like: — code start — and have a small program which just extracts these sections and put them in a file we can use at build time.