man-configuration.xml is the only manpage we build using docbook at this
time, and man-pages.xml includes just this one file. inline
man-configuration.nix into man-pages.xml so we can refer to a single
file to grab the manpage sources later rather than using find-by-suffix.
since we want to move away from docbook and having docbook manpages
around is going to block further progress we have to translate the
current nixos manpages from docbook it *something* else. mdoc seems the
most appropriate choice since markdown can't represent all the things
manpages can differentiate with even more extensions. if we ever need
html manpages we can render them using troff, like many of the online
manpage viewers, or rewrite them again. since we haven't had html
manpages for any of these in many years that seems unlikely to happen.
unlike most of the recent markdown conversions this comes with a lot of
minor rendering changes, mostly in spacing. docbook-xslt creates manual
pages in a very different dialect than mdoc (which is used here).
