Integrate the book well

[skade]

Currently, there's an iframe integration of the book into the page (currently not even the right book).

That does work surprisingly well, but it would be great if the book had a little bit of a theme.

The only necessary feature is that the header of the page stays visible.

[skade]

Currently, all "Learn" links just link to book.async.rs. It would be nice to have a proper rendered doc, like on https://rom-rb.org/learn