Update API Docs Sphinx Theme

As discussed in the TSC already, our present theme used in the API documentation is [Press Theme](https://github.com/schettino72/sphinx_press_theme), and it has unfortunately been marked as looking for a new maintainer. I'm sure someone will take up that request, but it won't be us.

Given the slightly uncertain future of the theme, and some visual oddities it can produce, its a good opportunity to revamp the theme to something like [Furo](https://github.com/pradyunsg/furo) which keeps a clean and modern aesthetic with much more community support for the project its self. We've also seen that fellow ASWF/O*IO project OpenImageIO has adopted it themselves.

In theory its a single line change in the Sphinx configuration, so may be a good first issue, but its not immediately clear if it will require larger changes to maintain readability across the docs.