The Elastic agent standalone reference manifests should document persisting the state path

[cmacknz]

To persist the state of the agent and integrations on a node the agent state path can be mounted to a host path persistent volume. Our ECK operator does this by default but I don't believe we document it in the standalone agent Kubernetes manifests.

We should:

• Document the STATE_PATH environment variable and the default path of /usr/share/elastic-agent/state
• Include mounting the STATE_PATH to a host path persistent volume claim in our reference self-managed K8S YAML. Persistent the state path almost always the desired behaviour, particularly when using integrations that ingest log files as the ingestion progress is check pointed to disk.

[jlind23]

Closing this as done by #4611