refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/administration/nixos-state.section.md

@@ -0,0 +1,28 @@
+# NixOS {#sec-nixos-state}
+
+## `/nix` {#sec-state-nix}
+
+NixOS needs the entirety of `/nix` to be persistent, as it includes:
+- `/nix/store`, which contains all the system's executables, libraries, and supporting data;
+- `/nix/var/nix`, which contains:
+  - the Nix daemon's database;
+  - roots whose transitive closure is preserved when garbage-collecting the Nix store;
+  - system-wide and per-user profiles.
+
+## `/boot` {#sec-state-boot}
+
+`/boot` should also be persistent, as it contains:
+- the kernel and initrd which the bootloader loads,
+- the bootloader's configuration, including the kernel's command-line which
+  determines the store path to use as system environment.
+
+
+## Users and groups {#sec-state-users}
+
+- `/var/lib/nixos` should persist: it holds state needed to generate stable
+  uids and gids for declaratively-managed users and groups, etc.
+- `users.mutableUsers` should be false, *or* the following files under `/etc`
+  should all persist:
+  - {manpage}`passwd(5)` and {manpage}`group(5)`,
+  - {manpage}`shadow(5)` and {manpage}`gshadow(5)`,
+  - {manpage}`subuid(5)` and {manpage}`subgid(5)`.