Make responsive layout for /book more responsive

I use a tiling window manager. My browser display width is 1042 pixels and I'm getting the mobile view because you require a 1060 pixel width. It'd make sense to use max-width instead of width on the body of the document so that you can drastically decrease the width requirement to display the sidebar.

[steveklabnik]

Hey, thanks for this report. It should go on the actual Rust repo though, so I've moved it: rust-lang/rust#22682