Setup readthedocs and automated builds

[seifertm]

After we can build a static documentation site (#408) we can upload it to readthedocs. The upload should be performed automatically as part of each release.

This involves setting up the readthedocs project and extending the current GitHub Actions pipeline to build the docs and upload them. The project setup on readthedocs needs to be performed by one of the maintainers.

The readthedocs tutorial may be helpful.

[seifertm]

Readthedocs has been set up.
See https://pytest-asyncio.readthedocs.io