feat: Consider "Hints" as a way to produce semantic 'Boxes'

The print edition will contain boxes as callouts. It would be good to have a semantic representation of this in the raw markdown content.

Hugo Book uses 'Hints':

https://themes.gohugo.io//theme/hugo-book/docs/shortcodes/hints/

These could work quite nicely. A good example of what might be a hint online (or a box in a book) would be the 'where does the name grep come from' in Chapter 12.