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/writing-documentation.chapter.md

@@ -0,0 +1,31 @@
+# Writing NixOS Documentation {#sec-writing-documentation}
+
+As NixOS grows, so too does the need for a catalogue and explanation of
+its extensive functionality. Collecting pertinent information from
+disparate sources and presenting it in an accessible style would be a
+worthy contribution to the project.
+
+## Building the Manual {#sec-writing-docs-building-the-manual}
+
+The sources of the [](#book-nixos-manual) are in the
+[`nixos/doc/manual`](https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual)
+subdirectory of the Nixpkgs repository.
+
+You can quickly validate your edits with `devmode`:
+
+```ShellSession
+$ cd /path/to/nixpkgs/nixos/doc/manual
+$ nix-shell
+[nix-shell:~]$ devmode
+```
+
+Once you are done making modifications to the manual, it's important to
+build it before committing. You can do that as follows:
+
+```ShellSession
+nix-build nixos/release.nix -A manual.x86_64-linux
+```
+
+When this command successfully finishes, it will tell you where the
+manual got generated. The HTML will be accessible through the `result`
+symlink at `./result/share/doc/nixos/index.html`.