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/configuration/overlayfs.section.md

@@ -0,0 +1,29 @@
+# Overlayfs {#sec-overlayfs}
+
+NixOS offers a convenient abstraction to create both read-only as well writable
+overlays.
+
+```nix
+{
+  fileSystems = {
+    "/writable-overlay" = {
+      overlay = {
+        lowerdir = [ writableOverlayLowerdir ];
+        upperdir = "/.rw-writable-overlay/upper";
+        workdir = "/.rw-writable-overlay/work";
+      };
+      # Mount the writable overlay in the initrd.
+      neededForBoot = true;
+    };
+    "/readonly-overlay".overlay.lowerdir = [
+      writableOverlayLowerdir
+      writableOverlayLowerdir2
+    ];
+  };
+}
+```
+
+If `upperdir` and `workdir` are not null, they will be created before the
+overlay is mounted.
+
+To mount an overlay as read-only, you need to provide at least two `lowerdir`s.