Update ReadTheDocs configuration

Since last time this was published, readthedocs (and everything else) has moved on - this updates the RTD config to get it building again.

I've set Sphinx to v6 in poetry, re-locked, and then frozen those requirements to use on readthedocs. I've also added a readthedocs config file, and explicitly included the project in its requirements.txt which ensures the package is available for sphinx-autoapi.