Improve docs/model_guide.md with skeleton template code + description of utils like Collator and Reorderer

[haileyschoelkopf]

As per title, some of the machinery we use in HFLM around the ordering, batching, and caching of requests is still opaque to most users.

It'd be great if we could expand model_guide.md with:

• in templates/new_template_model, a filestub containing the rough outline of a fully-featured HF-like local model impl that uses reorderer, collator, partial caching, and so on
• Content that explains key footguns / common mistakes when writing a new model class (/ a mini FAQ)
• more description and documentation of what the different request types are and what they do (perhaps turn this into a blogpost we can link out to instead?)