the footnote extra expects to only parse one document over the Markup's
lifetime, and writes the footnotes to the bottom of every page that is
rendered (again assuming only one) with links back to the reference
having one parser for the entire app, naturally, introduced
ever-increasing footnote links and every footnote on the site showing up
on every page. this was not intended
in some light testing, doing this per-request has a nominal effect on
performance
