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>

README.md

@@ -299,6 +299,12 @@ close · `SUPER+1..9` workspaces · `Print` region screenshot.
   the Nix modules. One place — there is no second renderer to keep in sync.
 - **Importing more old-distro palettes:**
   `tools/import-palettes.py <palettes-dir> themes/`.
+- **New opt-in feature (convention):** when a feature is off by default and
+  needs the user to set `nomarchy.*` options (e.g. night light, per-device
+  keyboard layouts, monitor layout, power management), ship a **commented**
+  example of it in `templates/downstream/home.nix` or `system.nix`. New
+  installs then enable it by uncommenting + tweaking, rather than hunting the
+  docs for the option name.
 
 ## Roadmap & known issues