readthedocs build failing due to no index.html generated

[hkershaw-brown]

On August 1st 2023 readthedocs deprecated builds that do not produce an index.html

https://blog.readthedocs.com/builds-without-index/

This causes the following error:

Note docs.dart.dart.edu is still up. This error is for new builds of the documentation - which is why pull requests are failing to build the docs.

Looking into the sphinx versions: master_doc is renamed root_doc from sphinx v4+ https://www.sphinx-doc.org/en/master/usage/configuration.html