[REQ] [Documentation] Customization Docs need a huge upgrade

Is your feature request related to a problem? Please describe.

It took ages to start understanding how the generator worked and there was basically no documentation so I was entering class after class to read and try to understand the codebase to customize the generator. There are also a lot of basic things that i didn t know about beforehand because they are just present as 1 line in a class and no doc is there for them.

Describe the solution you'd like

I think a general Customization doc for Generators/Codegens would be very helpfull. It should contain the order of opperation of the functions you can override, OpenAPIpreprocessing -> ModelGeneration -> APIGeneration, etc... maybe a bit more in detail. It should also contain the Normalizer Rules and what to add and etc... because these are not mentioned anywhere i read. there's a big need for clarity in general

No files or tests are identified in the issue. Start by tracing the customization flow from OpenAPI preprocessing through model generation and API generation, then review the normalizer rules. Done means a clear customization guide documents the operation order, override points, and normalizer guidance.