Document the contents & shapes of the NetCDF files that `nowcasting_dataset` outputs?

[JackKelly]

The Pydantic models in the data_sources/<modality>/<modality>_model.py files (where <modality> is one of {datetime, metadata, gsp, nwp, pv, satellite, sun, topographic}) describe the contents of the batches (and, hence, the contents of the NetCDF files). (For example, see satellite_model.py).

But it might be nice to aggregate all that documentation together into a single human-readable page somewhere? So it's really quick and easy for users to get an overview (and to see the detail) of precisely what nowcasting_dataset outputs to disk?

I guess the ideal situation would be to automatically extract the description fields from the pydantic models? I wonder if sphinx can do that?!

Related to #222