Doc: add Attributes section

[mrava87]

Closes #488.

This PR aims to update all Attributes sections of the docstrings of operators and solvers, including all of the internal attributes that we want users/devs to be aware of (previously we simply reported shape/explicit and missed many useful ones) - note that Parameters that are saved as class members are not added to the Attributes list, only new class members are.

Moreover, since the Attributes sections are now meaningful we make them appear in the Sphinx documentation.

Additional:

• some of the attributes that I feel should be kept hidden from users (to prevent anyone from tinkering with them) are now private (ie added _*). Ideally I would have like to be more aggressive but I was worried about breaking backward compatibility for some attributes that users may print or check so I decided to be very conservative.
• all np.ndarray in docstrings have been switched to numpy.ndarray to be able to have hyperlinks pointing to the NumPy documentation.
• all :obj:func in the documentation are turned into :obj:callable
• fixed all outstanding warnings in the Sphinx documentation build

[mrava87]

@cako this was long overdue and with some help with Copilot it took less than expected (I run out of the autocompletion allowance this months ahah).

I am not expecting you to look at this line by line, but let me know if you have any general suggestion 😄

[cako]

This is amazing!! Huge effort but will result in much better docs!! Thanks