Move to jupyter-book 2

[ocefpaf]

Let's list the issues we find here so we can try to address them later:

• The nb-gallery does not work;
• project/exclude files doesn't skip the archived notebooks;
• Page iframes are rendered as single pages;
• Run on external computing interface is broken. We need to restore the options for binder and google notebooks;
• Remove the thebe kernel button;

Closes #331