for the longest time we completely dropped link targets in
configuration.nix.5. let's stop doing this now and instead provide a
footnote for each link in a given option, numbered locally per option.
we will currently duplicate the link for <labelless-links> because it
makes it easier to get the collection of all links in a given option.
this may not be useful enough, so over time we might decide to drop the
footnotes for such links.
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