[docs] @auto_docstring decorator

[stevhliu]

updates the @auto_docstring docs as part of the process of improving the model addition docs

• updates title to be more precise as it can mean documenting a model in a model.md file as well
• flat hierarchy before → each component-type example has their own section now so its easier to find and reference. ### Documenting arguments is moved under ## Usage because it makes sense they live under one roof. how to document arguments is also part of the usage guidance
• modeling_model.py was treated as the default before and there was a separate section for modular files → reframe modular files as the default since this is the standard way to add models
• add some missing kwargs (BaseImageProcessor, ProcessorMixin, ModelOutputArgs)
• removed some redundancies
• update with some missing infos and adds additional context for some things like the @strict decorator
• replace ## Best practices with a quick do/don't checklist users can quickly scan and reference

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[zucchini-nlp]

lgtm

[zucchini-nlp]

oh nice, much better indeed

[yonigozlan]

Very nice, looks much cleaner! lgtm!