[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

Merge branch 'backport-mnowak/fix-dnssec-guide-footnote-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9671