Switch to Zola

Turns out the book theme has a bunch of limitations which add a few weird things to this MR.

Every page is in a section because the template does not handle orphan pages. This implies that the index page itself has to be a redirection to a page in a section (so I picked intro/_index.md). If I do not put a redirection in there, it just crashes.

I added a custom template to have a page's table of contents displayed in the sidebar when it is opened, to reduce the amount of scrolling required to truly find out what is in a page.