Make human readable version of schema

[aidanheerdegen]

@joshuatorrance has done a nice job with creating a tool to render the access-output-data-specifications schema to human readable docs:

https://access-output-data-specifications.readthedocs.io/en/latest/

It would be good to do something similar for our other schema.

I don't think it would be worthwhile making separate RTD sites for each schema, but perhaps it would be feasible to combine and organise them in some way?