Provide an overview of how turtle works

[sunjay]

Somewhere, in CONTRIBUTING.md or otherwise, we need to document the architecture of the turtle crate. That means talking about its dual-process design, how animations work, drawing commands, interprocess communication, etc.

If this information is not directly in CONTRIBUTING.md, it needs to be linked in that file.

This is very important for onboarding new contributors into the turtle codebase.

[sunjay]

This talk gives a good overview: https://youtu.be/ZtER1Wllbeo