Guide Structure

Ideas on how to structure the additional documentation here. Judging from https://github.com/RustPython/RustPython/issues/1715, people want an idea on how to embed and or extend RustPython. 

As such, a good template on which to base the structure of this guide would be something akin to [Python's guide for extending and embedding](https://docs.python.org/3/extending/index.html).

