refactor: move nixos-manual to reference/ + graphifyignore

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

@@ -0,0 +1,16 @@
+# ZFS {#sec-zfs-state}
+
+When using ZFS, `/etc/zfs/zpool.cache` should be persistent (or a symlink to a persistent
+location) as it is the default value for the `cachefile` [property](man:zpoolprops(7)).
+
+This cachefile is used on system startup to discover ZFS pools, so ZFS pools
+holding the `rootfs` and/or early-boot datasets such as `/nix` can be set to
+`cachefile=none`.
+
+In principle, if there are no other pools attached to the system, `zpool.cache`
+does not need to be persisted; it is however *strongly recommended* to persist
+it, in case additional pools are added later on, temporarily or permanently:
+
+While mishandling the cachefile does not lead to data loss by itself, it may
+cause zpools not to be imported during boot, and services may then write to a
+location where a dataset was expected to be mounted.