Update Sphinx to v6 once Read-the-Docs theme supports it #75
Description
Relevant information: https://blog.readthedocs.com/sphinx6-upgrade/
The latest version of sphinx breaks the read-the-docs theme. Currently, there is a pin in the environment.yml pinning sphinx <6 because of this. Once the sphinx-rtd-theme v1.2.0 comes out, it will support sphinx v6, so the pin can be removed.