[Helm] Missing volume mount in chart

Hi,

I noticed that the `volumeMounts` is missing when there is persistent volume usage within the StatefulSet: :

https://github.com/briefercloud/briefer/blob/main/chart/templates/jupyter-statefulset.yaml
```
        volumeMounts:
        - name: jupyter-storage
          mountPath: /home/jupyteruser
```

Without this, all saved data will be lost when the pod is restarted.