as far as we can tell nixos has only ever had a total of one olink, and
currently has no olinks at all. we can't currently represent olinks in
markdown docs, and if we re-add support for cross-document links they
will take a different form (and not use docbook, which will have to be
phased out before we re-add anything).
the olinkdb is thus unused and takes 10 seconds on our machine to build,
holding up the rest of the manual for no benefit.
617822c95b