refactor: split common/nixos.nix and -/home.nix

Split these big files into modular files that
are easier to share and read on their own.

profiles/common/nixos/networkdevices.nix

@@ -0,0 +1,30 @@
+{ pkgs, ... }:
+{
+  # enable scanners over network
+  hardware.sane = {
+    enable = true;
+    extraBackends = [ pkgs.sane-airscan ];
+  };
+
+  # enable discovery and usage of network devices (esp. printers)
+  services.printing = {
+    enable = true;
+    drivers = [
+      pkgs.gutenprint
+      pkgs.gutenprintBin
+    ];
+    browsedConf = ''
+      BrowseDNSSDSubTypes _cups,_print
+      BrowseLocalProtocols all
+      BrowseRemoteProtocols all
+      CreateIPPPrinterQueues All
+      BrowseProtocols all
+    '';
+  };
+
+  services.avahi = {
+    enable = true;
+    nssmdns4 = true;
+    openFirewall = true;
+  };
+}