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/ipv4-config.section.md

@@ -0,0 +1,41 @@
+# IPv4 Configuration {#sec-ipv4}
+
+By default, NixOS uses DHCP (specifically, `dhcpcd`) to automatically
+configure network interfaces. However, you can configure an interface
+manually as follows:
+
+```nix
+{
+  networking.interfaces.eth0.ipv4.addresses = [
+    {
+      address = "192.168.1.2";
+      prefixLength = 24;
+    }
+  ];
+}
+```
+
+Typically you'll also want to set a default gateway and set of name
+servers:
+
+```nix
+{
+  networking.defaultGateway = "192.168.1.1";
+  networking.nameservers = [ "8.8.8.8" ];
+}
+```
+
+::: {.note}
+Statically configured interfaces are set up by the systemd service
+`interface-name-cfg.service`. The default gateway and name server
+configuration is performed by `network-setup.service`.
+:::
+
+The host name is set using [](#opt-networking.hostName):
+
+```nix
+{ networking.hostName = "cartman"; }
+```
+
+The default host name is `nixos`. Set it to the empty string (`""`) to
+allow the DHCP server to provide the host name.