footnotes for rst2html5

The rst2html5 writer follows the old treatment of writing HTML footnotes,
but iiuc modern usage favors the use of an `aside` element (with ongoing
discussion of adding `note` and `noteref` elements).

Could the rst2html writer possibly move to the use of `aside`?

Background discussions:

- https://www.davidmacd.com/blog/html51-footnotes.html
- http://kb.daisy.org/publishing/docs/html/notes.html

Cheers,
Alan Isaac