Documentation for the "weights" module

It'd be great to get the code in the src/gdptools/weights subdirectory added to the hosted documentation, might just take the addition of:

.. automodule:: gdptools.weights.calc_weight_engines
    :members:

to the weight_get_classes.md file?

As an aside, it'd be great to have the contributing documentation include instructions on building the documentation locally, I'm not familiar with readthedocs or jupyter-book and the build pathway for the continuous integration jobs didn't have a clear line of code that builds the documentation somewhere in a pipeline.