New Docs

[kkinder]

There's a new docs site up on docs.puepy.dev. I switched from Writerside to mkdocs-material, which has some excellent features, and in addition to updated and improved documentation, a few new features:

• Demos now run inside the docs. You can see that here, for example: https://docs.puepy.dev/tutorial/04-refs/ -- notice that it shows running iframes of the code.
• Docstrings are now imported into a "references" section.

I'll probably want to add versioning with mike at some point, but mike's docs reference the old-school "gh-pages" branch system of deployment, and I'll have to sort out how that works with modern pages.