Ensure that no uncited footnotes exist in documents

[Atul Varma]

Discussing with @rtwell earlier this week, it became clear that we can provide a better UX if we simply disallow footnotes without citations.

Currently, the PDF parser allows uncited footnotes, simply adding them as direct children of the topmost policy node. But we should probably change this so it e.g. inserts them as paragraphs instead, or perhaps so it forcibly inserts citations at a relatively appropriate place.

Once we've done that, we should also build this concept into the validation, so e.g. API clients can't submit footnotes without citations.