kio-extras: enable man protocol

This installs the kio "man:" protocol handler, which fixes the UNIX manual
section in the KDE Help Center.

Note that kde currently parses "/etc/man.conf" manually, if `$MANPATH` is not
set, to build its man page index. (if https://bugs.kde.org/show_bug.cgi?id=404022
is addressed, the "/etc/man.conf" symlink should not be necessary anymore)

nixos/modules/misc/documentation.nix

@@ -156,6 +156,7 @@ in
       environment.systemPackages = [ pkgs.man-db ];
       environment.pathsToLink = [ "/share/man" ];
       environment.extraOutputsToInstall = [ "man" ] ++ optional cfg.dev.enable "devman";
+      environment.etc."man.conf".source = "${pkgs.man-db}/etc/man_db.conf";
     })
 
     (mkIf cfg.info.enable {