Add logging section for agent k8s standalone containers (#4234)

## Summary

Add a section to explain needed changes in order to log to a user
defined path when running the agent in a standalone k8s container.

## Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [ ] Yes  
- [x] No

---------

Co-authored-by: Karen Metts <[email protected]>

Author: michel-laterman

reference/fleet/running-on-kubernetes-standalone.md

@@ -266,4 +266,24 @@ If you are using Red Hat OpenShift, you need to specify additional settings in t
 
 Refer to [Kubernetes autodiscovery with {{agent}}](/reference/fleet/elastic-agent-kubernetes-autodiscovery.md) for more information.
 
+## Logging considerations
 
+Altering the default logging in a standalone container requires additional considerations. By default the {{agent}} logs to `stderr` and an internal destination so that diagnostics can be properly collected.
+
+
+To log to a custom filepath, make these changes to the manifest file:
+- {{agent}} logging must be configured in the configmap for `agent.yml`:
+    ```yaml
+    agent:
+      logging:
+        level: info
+        to_files: true  # Log to a custom filepath
+        to_stderr: true # Also log to stderr so that commands such as kubectl logs works as intended
+      files:
+          path: ${LOGS_PATH} # Use the env var to determine the logging path.
+    ```
+- The `LOGS_PATH` environment variable must be defined as a part of the Daeomonset's container specification.
+- The default DaemonSet container args must be changed to remove the `-e` option:
+    ```yaml
+    args: ["-c", "/etc/elastic-agent/agent.yml"]
+    ```