dev: NixOS 25.11 매뉴얼 로컬 사본 추가

nixpkgs nixos-25.11 브랜치의 nixos/doc/manual/ 디렉토리를 sparse clone으로
가져와 ~/obsidian/dev/nixos-manual/에 복사. _index.md에 구조/갱신 방법 정리.

오프라인 참조 + AI 에이전트 컨텍스트용. sandbox-tokyo 같은 NixOS 노드 운영 시
빠른 참조로 사용.

dev/nixos-manual/development/non-switchable-systems.section.md

@@ -0,0 +1,22 @@
+# Non Switchable Systems {#sec-non-switchable-system}
+
+In certain systems, most notably image based appliances, updates are handled
+outside the system. This means that you do not need to rebuild your
+configuration on the system itself anymore.
+
+If you want to build such a system, you can use the `image-based-appliance`
+profile:
+
+```nix
+{ modulesPath, ... }:
+{
+  imports = [ "${modulesPath}/profiles/image-based-appliance.nix" ];
+}
+```
+
+The most notable deviation of this profile from a standard NixOS configuration
+is that after building it, you cannot switch *to* the configuration anymore.
+The profile sets `config.system.switch.enable = false;`, which excludes
+`switch-to-configuration`, the central script called by `nixos-rebuild`, from
+your system. Removing this script makes the image lighter and slightly more
+secure.