switch to the Charter font

This was copied from the upstream .zip file as available here:

https://practicaltypography.com/charter.html

With the following command:

     find charter/ -name '*webfont*' -type f -name '*.eot' -o -name '*.woff' | xargs cp -t ~/src/ikiwiki-bootstrap-anarcat/fonts

Charter is fairly small: all the files committed here sum up to around
352K, and individual WOFF files are around 32KB, less than the
Javascript loaded in this site. And those are only loaded once,
possibly not at all if the font is already defined

See also https://anarc.at/blog/2020-03-10-font-changes