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.
This method name is somewhat misleading, since it includes replacing the search term too. How about something like:

```ruby
def heading_tag
  '<h2 id="footnotes">Footnotes</h2>'
end

def body
  document.body.gsub(footnote_open_tag, "#{heading_tag}\\&")
end
```