Add support for footnotes (?) (at least "footnotes to tables")

[thomasheritage]

I don't need footnotes in an html-pub document at the moment, but I am using "footnotes to tables" in an Engineering Document being authored using Word (based on feedback from @ERyan71258) so these will be needed in html-pub at some point.

Footnotes are covered in the ISO directives at https://www.iso.org/sites/directives/current/part2/index.xhtml#_idTextAnchor362 and "Footnotes to tables" are at https://www.iso.org/sites/directives/current/part2/index.xhtml#_idTextAnchor437

[thomasheritage]

Related to #291

[SteveLLamb]

Can you provide the example of the word document of this? Similar to #374, we hadn't coded this in, as we had no use case (and we certainly don't footnotes in prose with no "pages" in HTML).

[ERyan71258]

Please see #374.