Document how/when to use Cython in the codebase

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1391)
In GitLab by [[Gitlab user @BenjaminSchubert]](https://gitlab.com/BenjaminSchubert) on Aug 30, 2020, 11:08

## Summary

[//]: # (Summarize the bug encountered concisely)

Our hacking guide doesn't contain anything about how and when to use Cython in BuildStream,
we should have something about it.

It should at least contain the following:

- how to add a new cython module (where to register it, etc)
- Need a corresponding .pyi file for Mypy type checking
- best practices?

Start with the existing hacking guide and inspect how Cython modules are currently registered in BuildStream. Document how to add a module, the corresponding .pyi file required for Mypy type checking, and any established best practices. The work is done when the guide covers these points clearly for future contributors.

Written by the indexing model from the issue text.