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/administration/rebooting.chapter.md

@@ -0,0 +1,30 @@
+# Rebooting and Shutting Down {#sec-rebooting}
+
+The system can be shut down (and automatically powered off) by doing:
+
+```ShellSession
+# shutdown
+```
+
+This is equivalent to running `systemctl poweroff`.
+
+To reboot the system, run
+
+```ShellSession
+# reboot
+```
+
+which is equivalent to `systemctl reboot`. Alternatively, you can
+quickly reboot the system using `kexec`, which bypasses the BIOS by
+directly loading the new kernel into memory:
+
+```ShellSession
+# systemctl kexec
+```
+
+The machine can be suspended to RAM (if supported) using `systemctl suspend`,
+and suspended to disk using `systemctl hibernate`.
+
+These commands can be run by any user who is logged in locally, i.e. on
+a virtual console or in X11; otherwise, the user is asked for
+authentication.