Section on "Overall Book Design" is outdated and needs repair

[daobrien]

https://stylepedia.net/style/6.0/#overall-book-design

If you read through the subsections here it's easy to find content that is
outdated or unclear. I think this section was written before the guide went
public, and it shows. Some of the content is still fine; it just needs
"modernizing."

This is not a big section so shouldn't take too long.