docs(template): ship opt-in features as commented examples downstream

Convention: an off-by-default feature that needs nomarchy.* settings in the
downstream files should appear commented-out in templates/downstream/{home,
system}.nix, so a new install enables it by uncommenting + tweaking rather
than hunting the docs for the option name. Apply it now: night light,
nomarchy.monitors, per-device keyboard layouts (home.nix) and power
management (system.nix). Also fix the stale keyboard.layout note
(console/LUKS follow xkb automatically now). Documented in README S6.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/downstream/home.nix

@@ -5,13 +5,34 @@
 { pkgs, ... }:
 
 {
-  # Examples:
+  # ── Overrides (the desktop is on by default; tweak or opt out) ──────
   # nomarchy.terminal = "kitty";        # swap the default terminal
   # nomarchy.waybar.enable = false;     # bring your own bar
   # nomarchy.stylix.enable = false;     # opt out of GTK/Qt theming
-  # nomarchy.keyboard.layout = "de";    # session keyboard (pair with
-  #                                     # xkb + console.useXkbConfig in
-  #                                     # system.nix for tty/LUKS)
+  # nomarchy.keyboard.layout = "de";    # session keyboard (set the matching
+  #                                     # services.xserver.xkb.layout in
+  #                                     # system.nix too; the console + LUKS
+  #                                     # prompt follow it automatically)
+
+  # ── Opt-in features — uncomment and tweak to enable ─────────────────
+  # nomarchy.nightlight = {             # scheduled blue-light filter (hyprsunset)
+  #   enable = true;
+  #   temperature = 4000;               #   night warmth in K — lower is warmer
+  #   sunset = "20:00";
+  #   sunrise = "07:00";
+  # };
+  #
+  # nomarchy.monitors = [               # declarative per-output layout, hotplug-applied
+  #   { name = "eDP-1"; position = "0x0"; }
+  #   { name = "HDMI-A-1"; position = "auto-right"; }
+  # ];                                  #   names from `hyprctl devices`
+  #
+  # nomarchy.keyboard.devices = {       # a specific keyboard's own layout
+  #   "keychron-keychron-k2" = { layout = "de"; };
+  # };
+  #
+  # nomarchy.keyboard.layouts = [ "de" "fr" ]; # rofi-prompt for a layout when a
+  #                                            # new keyboard connects + remember it
 
   home.packages = with pkgs; [
     # firefox