[9.18] fix: doc: All footnotes need to have unique name and be referenced
With Sphinx 8.1.0, footnotes can't stand on their own and have to be referenced from somewhere, otherwise build fails, e.g.:
doc/dnssec-guide/signing.rst:1470: WARNING: Footnote [#] is not referenced. [ref.footnote]
Backport of MR !9663 (merged)