Fix publishing to Read the docs

[seddonym]

Documentation releases are broken.

[seddonym]

This is now fixed, but the more recent versions are missing docs. I'm inclined to leave this as I'm not sure how to trigger a build on the tags without also triggering a release on PyPI (which also uses tags).

To mitigate this I've changed the RTD default branch to latest - I'll change this back to stable once I do a new release.

Leaving this ticket open to remind myself to do that.

[seddonym]

This is now done.