Add a <h2> for footnotes in manual documents

[Trello ticket](https://trello.com/c/oiJYOedK/266-manuals-footnotes-frontend)

This allows manuals-frontend to correctly render an extra 'collapsible section' for footnotes.

No manuals are currently published, so we won't need to republish anything.
I'm not wild about this classname. It makes it sound like it's rendering govspeak footnotes in the body. I don't have a good suggestion right now, but something we should chat about next week.