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/installation/installing-pxe.section.md

@@ -0,0 +1,33 @@
+# Booting from the "netboot" media (PXE) {#sec-booting-from-pxe}
+
+Advanced users may wish to install NixOS using an existing PXE or iPXE
+setup.
+
+These instructions assume that you have an existing PXE or iPXE
+infrastructure and want to add the NixOS installer as another
+option. To build the necessary files from your current version of nixpkgs,
+you can run:
+
+```ShellSession
+nix-build -A netboot.x86_64-linux '<nixpkgs/nixos/release.nix>'
+```
+
+This will create a `result` directory containing:
+
+* `bzImage` -- the Linux kernel
+* `initrd` -- the initrd file
+* `netboot.ipxe` -- an example ipxe script demonstrating the appropriate kernel command line arguments for this image
+
+If you're using plain PXE, configure your boot loader to use the
+`bzImage` and `initrd` files and have it provide the same kernel command
+line arguments found in `netboot.ipxe`.
+
+If you're using iPXE, depending on how your HTTP/FTP/etc. server is
+configured you may be able to use `netboot.ipxe` unmodified, or you may
+need to update the paths to the files to match your server's directory
+layout.
+
+In the future we may begin making these files available as build
+products from hydra at which point we will update this documentation
+with instructions on how to obtain them either for placing on a
+dedicated TFTP server or to boot them directly over the internet.