Documentation for the interpreter

[mahrud]

Macaulay2 packages are decently documented, the Core has documentation (except everything in #1668), and the engine has some documentation. As far as I'm aware the interpreter code in the d/ directory and the translator in the c/ directory have no documentation. Not only that, since the language is made up, there's no external way to learn the language. Needless to say, this is a problem that will only get worse if it isn't documented.

There are now M2internals meeting notes about the language itself, and a few aspects of the interpreter, but what should be the preferred way to document the sources in the interpreter?

Ideally, I wish there was a roadmap for cleaning up that directory and documenting it along the way.