[Experimental] Use jupyter-book 2.0 for building the lectures

[kp992]

Experimental setup for building lectures with jupyter-book 2.0.

This PR is built on top of #345.

[github-actions]

🚀 Deployed on https://688038a10dbee1dd2ed3075b--epic-agnesi-957267.netlify.app

[mmcky]

@kp992 thanks for opening this. Excellent work! It is exciting to see how close we are now.

From my review it looks like there a just a few issues and features we need before launch.

Bugs:

1. LHS TOC menu flickers when using the Home page or a top level Part section. It is like it has an auto-stow feature that is triggering when pages are reloaded. It doesn't look great when pages are refreshed.
2. Code block formatting has too much indentation for Solution blocks

Features:

1. I see a bunch of new export notebooks being added to the yaml headers of each lecture. Do we need to do some development work on supporting the download notebook link in the theme?
2. Is index support on the jb2 / mystmd roadmap?