Document solver interface

[bagibence]

Document the why and how of the solver interface in the developer notes.

• Mention the reason for moving away from JAXopt.
• Add class hierarchy.
• Add instructions for writing a new solver.
• Include suggestions for interfacing with existing optimization libraries.
• Remove JAXopt-specific text.

[bagibence]

• Clean up the text in the developer notes
• Add a how-to guide, demonstrating how to make a solver that adheres to the API

[bagibence]

@BalzaniEdoardo @billbrod I added a how-to guide for creating a custom solver as part of #365.

I'm not sure what the workflow is for making docs, so I made custom_solvers.md with jupytext from an ipynb, then edited the kernel metadata to be the same as for the other notebooks by hand.
Please let me know if there is a better way you guys use.