Dedicated documentation pages

[TomNicholas]

For now the documentation on how to use this package has just been written into the README, but as we add more features (see #1), we should move to a dedicated documentation page.

Conventionally this is done using sphinx to generate html files from ReStructuredText (.rst) files, and hosted via readthedocs. For example the xarray project stores its many pages of documentation here, which is then accessible at https://docs.xarray.dev/en/stable/. I'm wondering if we might want to use MyST instead so that we can write our documentation as markdown (.md) files instead, which have a nicer syntax than .rst files (and use the same syntax as used in all github issues and comments!)

[NoraLoose]

Is setting up a dedicated documentation of low or high priority right now? @joegaines