Improve Documentation

[kmheckel]

The current documentation is passable but it could always be better.

• [x] Make spyx.nn listings cleaner by removing unwanted special-members
• [x] Incorporate notebooks from the examples/research folders.
• [ ] More math equations for surrogate gradient functions in spyx.axn
• [ ] Write intro and quickstart pages
• [x] Add documentation/tutorial for NIR exporting beyond the sample notebooks.demo
• [ ] Add better return type annotations showing the arguments for callables returned by HOFs/function builders.