Misc fixes needed to build docs locally

[dstansby]

Does what it says in the title.

To test locally:

python -m pip install -e ".[docs]"
cd docs
make html

The HTML docs should then be built succesfully in the _build/html directory.

[samcunliffe]

🍉

Tested locally, works.

[samcunliffe]

Just to confirm, this all now comes from docs/api.rst right? The automodapi directive handles all of this for us.

I never used the sphinx-automodapi extension (and it looks like it would have saved me a lot of time).

[samcunliffe]

Astrophysicists makin' cool toys.

[dstansby]

Yes, that's right. This should never have been committed 🙈