Consolidate and expand Developer Documentation

[Giuseppe5]

Is your feature request related to a problem? Please describe.
It is not always easy to familiarize with the Brevitas codebase.
Among other things, because of dependency it is not immediately clear how the different element combine together and the debugger might not always help.
Similarly, we would need a better explanation of the hierarchical structure, the export structure, the working principles behind the implementation of our PTQ algorithms, and so on..

Describe the solution you'd like
We have some documents/notebooks explaining some architectural decisions of Brevitas, but it might be worth collating them together in a more coherent fashion, and going in more gradual steps to allow new developers a better understanding of the core aspects of Brevitas.

This doesn't have to be done all at once, and this issue is here mostly to capture the general concept of the task, to be further divided.

Thanks @rk119 for inspiring this