Improve compatibility with common sphinx HTML themes

[chrisjsewell]

On looking into improving our documentation, I was seeing if we could move to the furo theme https://github.com/pradyunsg/furo, since these days it is probably the most widely used sphinx theme.

I have found this quite difficult to get looking nice with the current HTML/CSS of sphinx-needs.
As exampled by the screenshot below, you can see that our current documentation relies on quite a "delicate" mixture of CSS across multiple sources; certainly non-trivial for users to get correct

Basically, we should aim to support out-the-box some of the common sphinx themes (or at least have documentation on the CSS required).

Note, in sphinx-design I actually build the documentation for multiple themes (in synced repo branches). It may be overkill here, but it is a possibility: