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/preface.md

@@ -0,0 +1,11 @@
+# Preface {#preface}
+
+This manual describes how to install, use and extend NixOS, a Linux distribution based on the purely functional package management system [Nix](https://nixos.org/nix), that is composed using modules and packages defined in the [Nixpkgs](https://nixos.org/nixpkgs) project.
+
+Additional information regarding the Nix package manager and the Nixpkgs project can be found in respectively the [Nix manual](https://nixos.org/nix/manual) and the [Nixpkgs manual](https://nixos.org/nixpkgs/manual).
+
+If you encounter problems, please report them on the [`Discourse`](https://discourse.nixos.org), the [Matrix room](https://matrix.to/#/%23nix:nixos.org), or on the [`#nixos` channel on Libera.Chat](irc://irc.libera.chat/#nixos). Alternatively, consider [contributing to this manual](#chap-contributing). Bugs should be reported in [NixOS’ GitHub issue tracker](https://github.com/NixOS/nixpkgs/issues).
+
+::: {.note}
+Commands prefixed with `#` have to be run as root, either requiring to login as root user or temporarily switching to it using `sudo` for example.
+:::