[docs] Document cu2qu library
Reorganise the documentation so that everything is in one place and users are more clearly pointed to the modules which are likely to be useful for their purposes. (I still think it’s worth having at least a brief reference to ``cu2qu.cli`` in there, as a way of reminding users that there is a command-line implementation.) Docstrings are provided for non-API methods where I could understand them - trusting these will be useful for future maintainers.
