docs: updated zfs snapshotter support for k3s

  boot.kernelModules = [ "rbd" ];

```

## ZFS Snapshot Support

## ZFS ContainerD Support

K3s's builtin containerd does not support the zfs snapshotter. However, it is possible to configure it to use an external containerd:

The [ZFS snapshotter](https://github.com/containerd/zfs) can be enabled for k3s' embedded ContainerD though it requires mounting a dataset to a specific path used by k3s: `/var/lib/rancher/k3s/agent/containerd/io.containerd.snapshotter.v1.zfs`

For example:

```bash

$ zfs create -o mountpoint=/var/lib/rancher/k3s/agent/containerd/io.containerd.snapshotter.v1.zfs <zpool name>/containerd

```

virtualisation.containerd = {

  enable = true;

  settings =

    let

      fullCNIPlugins = pkgs.buildEnv {

        name = "full-cni";

        paths = with pkgs;[

          cni-plugins

          cni-plugin-flannel

        ];

      };

    in {

      plugins."io.containerd.grpc.v1.cri".cni = {

        bin_dir = "${fullCNIPlugins}/bin";

        conf_dir = "/var/lib/rancher/k3s/agent/etc/cni/net.d/";

      };

      # Optionally set private registry credentials here instead of using /etc/rancher/k3s/registries.yaml

      # plugins."io.containerd.grpc.v1.cri".registry.configs."registry.example.com".auth = {

      #  username = "";

      #  password = "";

      # };

    };

};

# TODO describe how to enable zfs snapshotter in containerd

services.k3s.extraFlags = toString [

  "--container-runtime-endpoint unix:///run/containerd/containerd.sock"

You can now configure k3s to use zfs by passing the `--snapshotter` flag.

```

services.k3s = {

  ...

  extraFlags = [

    "--snapshotter=zfs"

  ];

```