doc: add an example of system integration

deploy/helm/elastic-agent/examples/system-custom-auth-paths/README.md

@@ -0,0 +1,25 @@
+# Example: System Integration with custom auth log paths
+
+In this example we install the built-in `system` integration and specify custom paths for the auth logs stream (see [agent-system-values.yaml](agent-system-values.yaml)).
+
+## Prerequisites:
+1. A k8s secret that contains the connection details to an Elasticsearch cluster such as the URL and the API key ([Kibana - Creating API Keys](https://www.elastic.co/guide/en/kibana/current/api-keys.html)):
+    ```console
+    kubectl create secret generic es-api-secret \
+       --from-literal=api_key=... \
+       --from-literal=url=...
+    ```
+
+2. `system` integration assets installed through Kibana ([Kibana - Install and uninstall Elastic Agent integration assets](https://www.elastic.co/guide/en/fleet/current/install-uninstall-integration-assets.html))
+
+## Run:
+```console
+helm install elastic-agent ../../ \
+     -f ./agent-system-values.yaml \
+     --set outputs.default.type=ESSecretAuthAPI \
+     --set outputs.default.secretName=es-api-secret
+```
+
+## Validate:
+
+1. The Kibana `system`-related dashboards should start showing up the respective info.

deploy/helm/elastic-agent/examples/system-custom-auth-paths/agent-system-values.yaml

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+system:
+  enabled: true
+  authLogs:
+    vars:
+      paths:
+        - /var/log/custom_auth.log
+
+agent:
+  unprivileged: true