[docs] Add a codemap to help find your way around the code

Describes the layout of this repo. This should help contributors navigate and and find what they're looking for much faster than just stumbling around.

I have experience with both contributing to and maintaining open-source projects. One of the lessons I’ve learned is that the biggest difference between an occasional contributor and a core developer lies in the knowledge about the physical architecture of the project. Roughly, it takes 2x more time to write a patch if you are unfamiliar with the project, but it takes 10x more time to figure out where you should change the code.

https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html

Also update the README and remove the outdated HACKING.