mdbook: move font decls to variable files

Currently we overload mdbook's fonts.css file to declare all the `@fontfaces` all of our themes could want, but we don't vary the fonts by theme.

We should either:

* not override this file, and instead place fontface decls in the variable files that want them (axo.css, cupcake.css, ...)
* template this file and write the fontface decls in dynamically

I'm more inclined to the first option, as it keeps more things in one place.

Start by reading fonts.css and the theme variable files named in the issue, including axo.css and cupcake.css. Compare where the current @font-face declarations are defined and determine whether the preferred per-theme approach fits the existing structure. Done means themes no longer share font declarations they do not use, with the selected approach applied consistently.

Written by the indexing model from the issue text.