A reference to a footnote, in the text (type "footnotereference"):
Markdown's [^label]. The JSON key is label, the label of the
Kotoba.Nodes.FootnoteDefinition that it refers to.
In HTML, a reference is the number of its footnote, in a sup, linked to
the footnote. Footnotes are numbered in the order of their first
reference. In plain text and in Markdown, it is [^label].