Set up ReadTheDocs documentation

[dlqqq]

It's time to get documentation set up on ReadTheDocs. The boilerplate can be generated with sphinx-quickstart docs, as described in the documentation. However, there will need to be a few additional changes to this:

• Use myst-parser to accept Markdown source
• Use pydata-sphinx-theme

Please review this for inspiration: jupyter-server/jupyter-scheduler#311