Finish chapter on type checking

[rylev]

The type checking chapter is not finished, and does not even really talk about the process of type checking.

[younes-io]

I think this has the merit to be documented as a priority

[jyn514]

https://sdleffler.github.io/RustTypeSystemTuringComplete/ has some interesting details about how unification works fwiw

[llooFlashooll]

Hi, I think the discussion about Rust type checking is blurred. I am willing to participate in completing the related doc.