Vyper: Documentation for compiler API

[fubuloubu]

It would be nice to have more documentation (and doctests) for the compiler API. This is endpoint that will support other tools, so it should really clear how to interact with it and what the options are. Also, if we allow deeper integration into the compilation stages, this will be really important to document what each stage takes as input and output.

[jacqueswww]

Good idea, I would just make this a "Integrating Vyper" section.

[iamdefinitelyahuman]

We have https://github.com/vyperlang/vyper/tree/master/vyper/compiler and the docstrings for compile_code and compile_codes are fairly extensive. Should we still consider a section in the actual documentation, or can we consider this good enough?