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/network-problems.section.md

@@ -0,0 +1,21 @@
+# Network Problems {#sec-nix-network-issues}
+
+Nix uses a so-called *binary cache* to optimise building a package from
+source into downloading it as a pre-built binary. That is, whenever a
+command like `nixos-rebuild` needs a path in the Nix store, Nix will try
+to download that path from the Internet rather than build it from
+source. The default binary cache is `https://cache.nixos.org/`. If this
+cache is unreachable, Nix operations may take a long time due to HTTP
+connection timeouts. You can disable the use of the binary cache by
+adding `--option use-binary-caches false`, e.g.
+
+```ShellSession
+# nixos-rebuild switch --option use-binary-caches false
+```
+
+If you have an alternative binary cache at your disposal, you can use it
+instead:
+
+```ShellSession
+# nixos-rebuild switch --option binary-caches http://my-cache.example.org/
+```