Docs updates

[biphasic]

Made some changes to the sphinx documentation:

• Used Sphinx book theme. This theme looks more modern and gives you a lot more options than the standard theme
• Used sphinx-autoapi for nice API reference generation
• Moved example notebooks to docs with the help of Myst-nb. Currently they are not being executed as part of the documentation generation process, but once they've been restructured a bit, this could be a good option to make sure that the tutorial code always works.
• Used Sphinx Viewcode to include source code in docs for classes/functions

[kmheckel]

@biphasic The docs fail to build since autoapi isn't installed -- where do I need to add it for things to work?