I think some key documentation is missing for developers who are working on the MODX core. Therefore I think it would be handy to perhaps create a MODX Core development manual, add a short description in the repo README or to extend the becoming a contributor documentation (https://docs.modx.com/community/contribute/becoming-a-contributor) depending on the amount of documentation that will be added.
I think at least the following subjects should be documented somewhere to make it easier for developers to work on the core of MODX.
Where do you guys think this documentation should be added and can you think of any other subjects that might need to be documented for MODX core developers?
The MODX.org site that we're drafting content for has a contribution section, but for in-depth instructions and examples I think the documentation is still the best place.
I think adding a section Core development at the docs here would be logic and quite easy to find:
https://docs.modx.com/revolution/2.x/
@Ibochkarev Have a look, please. If you can create a skeleton of such a document, I will contribute and fill in the details.
@sdrenth https://docs.modx.com/current/en/contribute/code

This section has been seriously revised and rewritten @Mark-H , so I vote to close this request.
Yep, it covers the main topic and it can be improved, but the issue solved.
Most helpful comment
I think adding a section Core development at the docs here would be logic and quite easy to find:
https://docs.modx.com/revolution/2.x/