Scaffold nikita-nixos host

Add a second nixosConfigurations entry for a friend's machine running
Plasma 6 + SDDM. The hardware-configuration.nix is a placeholder; it
must be replaced with the output of nixos-generate-config on the
actual hardware before installing. Git identity in hosts/nikita-nixos/
home.nix is also a placeholder.

system/plasma.nix

@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  services.desktopManager.plasma6.enable = true;
+}