Handle footnotes in manual sections

[Trello ticket](https://trello.com/c/oiJYOedK/266-manuals-footnotes-frontend)

Manual sections containing footnotes now have an h2 added for the footnotes detail at the bottom of the body.

Style it appropriately.

When a user clicks on a footnote link, open the footnotes section so that the footnote content is visible.

(JavaScript with @alicebartlett)