mdoc is just too slow to render on groff, and semantic markup doesn't
help us any for generated pages.
this produces a lot of changes to configuration.nix.5, but only few
rendering changes. most of those seem to be place losing a space where
docbook emitted roff code that did not faithfully represent the input
text, though a few places also gained space where docbook dropped them.
notably we also don't need the compatibility code docbook-xsl emitted
because that problem was fixed over a decade ago.
this will handle block quotes, which the docbook stylesheets turned into
a mess of roff requests that ended up showing up in the output instead
of being processed.
10a4f0daca