[docs] Modular diffusers

[stevhliu]

Draft for a quickstart that should ideally briefly summarize everything a developer needs to know about Modular Diffusers without referencing other resources.

edit: expanding scope into other modular docs as well with additions like API references

(bit of a brain dump at the moment, not ready for review yet!)

[HuggingFaceDocBuilderDev]

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[stevhliu]

Ok, I think I have an initial version of the refactored modular docs @huggingface/diffusers!

• Added an API section for modular here. Let me know if there is anything useful that is missing!
• A bit undecided about the Quickstart/End-to-end doc at the moment. The Quickstart tries cover everything through the lens of implementing Differential Diffusion. The End-to-end example focuses more on a 4-step process for implementing Differential Diffusion. I think the Quickstart is more comprehensive but I'm not sure and would appreciate your feedback!
• Split Guiders out into a separate doc.
• There were many practical examples scattered through that I have omitted for the time being because they made the docs really long. I'm thinking of adding a "Recipes" section to showcase these practical examples.