refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/administration/containers.chapter.md

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+# Container Management {#ch-containers}
+
+NixOS allows you to easily run other NixOS instances as *containers*.
+Containers are a light-weight approach to virtualisation that runs
+software in the container at the same speed as in the host system. NixOS
+containers share the Nix store of the host, making container creation
+very efficient.
+
+::: {.warning}
+Currently, NixOS containers are not perfectly isolated from the host
+system. This means that a user with root access to the container can do
+things that affect the host. So you should not give container root
+access to untrusted users.
+:::
+
+NixOS containers can be created in two ways: imperatively, using the
+command `nixos-container`, and declaratively, by specifying them in your
+`configuration.nix`. The declarative approach implies that containers
+get upgraded along with your host system when you run `nixos-rebuild`,
+which is often not what you want. By contrast, in the imperative
+approach, containers are configured and updated independently from the
+host system.
+
+```{=include=} sections
+imperative-containers.section.md
+declarative-containers.section.md
+container-networking.section.md
+```