Improve Message Generation Documentation

[maspe36]

Currently, our message generation pipeline is quite complex. Its complex, mainly because Rust does not have a stable ABI, and wants to compile all of the source code and dependencies source code, when compiling a crate.

We need to start documenting all of the various repositories and their responsibilities as it pertains to our message generation, in one place, in this repo.

I have some local notes I can upload in a few days that could potentially be used as a starting point.

[maspe36]

There is a really nice starting point being added in https://github.com/ros2-rust/ros2_rust/pull/417. We should expand this to cover the various build tools and their responsibilities with actually building the generated rust message code.