Include function usage examples in docstrings/API documentation

[gamatos]

It would be nice to have small usage examples for specific functions such as is done e.g. here or here. These examples would be included either directly in the docstring or in the corresponding documentation page.

For instance, get_params_to_statetensor_func could have examples of what happens when no parameterised gates are included in the circuit specification, how to supply custom operations (parameterised and non-parameterised), etc.

Most functions in qujax are currently in need of such examples.