The text of a footnote (type "footnotedefinition"): Markdown's
[^label]: text. The JSON key is label. It holds blocks, and is valid
only under the root.
In HTML, the footnotes of a document are one list at its end
(section.kotoba-footnotes), numbered in the order of their first
reference (see Kotoba.Nodes.FootnoteReference); a footnote with no
reference comes last. In Markdown, a footnote is [^label]: and its
blocks, the blocks after the first indented by four spaces.