Migrate reference documentation to readthedocs

[sbesson]

The current strategy of capturing the full reference documentation of this library/plugin has some scaling issues e.g. as the documentation of the default context tries to cover the supported scenarios more extensively.

Similarly to what has been done in https://github.com/ome/ome-zarr-py with https://ome-zarr.readthedocs.io/en/stable/, we might consider migrating and supplementating the existing content using readthedocs. The README would still contain the quickstart documentation but include pointers to the reference documentation for detailed information.