docs: add k3s tip on auto-deploying manifests

docs/content/tips-n-tricks/k3s/_index.md

@@ -0,0 +1,29 @@
+The k3s project supports [automatic resource deployment](https://docs.k3s.io/installation/packaged-components#auto-deploying-manifests-addons) of files in the `/var/lib/rancher/k3s/server/manifests` directory.
+
+As such, on a server node, we can write kubenix's output there.
+
+```nix
+{
+  # let's write `resultYAML` to an arbitrary file under `/etc`
+  environment.etc."kubenix.yaml".source = 
+  (kubenix.evalModules.x86_64-linux {
+    module = { kubenix, ... }: {
+      imports = [ kubenix.modules.k8s ];
+      kubernetes.resources.pods.example.spec.containers.example.image = "nginx";
+    };
+  }).config.kubernetes.resultYAML;
+
+  # now we can link our file into the appropriate directory
+  # and k3s will handle the rest
+  system.activationScripts.kubenix.text = ''
+    ln -sf /etc/kubenix.yaml /var/lib/rancher/k3s/server/manifests/kubenix.yaml
+  '';
+}
+```
+{{< hint danger >}}
+**WARN**: this will write all manifests to the nix store and is therefore not suitable for inline sensitive data.
+{{< /hint >}}
+
+{{< hint info >}}
+**NOTE**: k3s will not delete resources if files are removed.
+{{< /hint >}}