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.
How about `FootnotesSectionHeadingRenderer`?