docs: add v9.0.0 migration guide with CSI NFS Driver (PSCLOUD-2)

docs/user/MigrationSteps-v9.md

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+
+#  Migration Guide: v9.0.0
+
+This guide assumes you are migrating a Viya deployment (e.g., `v8.2.1`) to a newer version (e.g., `v9.0.0`) using the latest DaC baseline that includes the `csi-driver-nfs`.
+
+##  Prerequisites
+
+- Ensure you have **cluster admin access**.
+- The **NFS server** used in the existing setup must be retained and accessible.
+- All **PVs and PVCs** should be backed up as a precaution.
+
+##  Migration Steps
+
+###  Backup Existing Viya Environment (Manual Trigger)
+
+Trigger a manual backup of your running Viya deployment:
+
+```bash
+kubectl create job --from=cronjob/sas-scheduled-backup-all-sources manual-backup-$(date +%s) -n <viya-namespace>
+````
+
+Fatch the backup ID
+
+```bash
+kubectl describe job <backup-job-name> -n va-viya | grep "sas.com/sas-backup-id"
+````
+
+###  Verify the backup job has completed successfully:
+
+```bash
+kubectl get jobs \
+  -l "sas.com/sas-backup-id=<backup-id>" \
+  -L "sas.com/sas-backup-id,sas.com/backup-job-type,sas.com/sas-backup-job-status,sas.com/backup-persistence-status"
+```
+###  Stop the Viya Deployment
+
+Stop the SAS Viya environment using the cron job:
+
+```bash
+kubectl -n <viya-namespace> create job --from=cronjob/sas-stop-all stopdep-<date +%s>
+```
+
+**Example:**
+
+```bash
+kubectl -n viya4 create job --from=cronjob/sas-stop-all stopdep-22072025
+```
+###  Delete Old NFS Provisioner Components
+
+Remove the `sas` StorageClass:
+For SAS Viya environments deployed on Google Cloud Platform (GCP), the legacy `pg-storage` StorageClass must be deleted.
+
+```bash
+kubectl delete storageclass sas
+```
+
+Delete the namespace used by the legacy provisioner (typically `nfs-client`):
+
+```bash
+kubectl delete namespace nfs-client
+```
+
+###  Deploy New Viya Environment with CSI Driver
+
+If you have redeployed SAS Viya using the updated DaC baseline that includes CSI NFS driver support, no additional action is required.
+
+However, if you have only updated the DaC baseline without redeploying Viya, you will need to manually start the Viya environment using the following command:
+
+```bash
+kubectl -n <viya-namespace> create job --from=cronjob/sas-start-all startdep-<date +%s>
+```
+
+>  **Important Note:** You do **not** need to restore from backup, as the NFS server path to the PVs remains the same. The CSI driver will reuse existing PVs and directories automatically.
+
+###  Post-Migration Steps
+
+*  Confirm all PVCs are **bound and mounted correctly** in the new Viya deployment.
+*  Validate **data availability** and application functionality.
+
+---
+
+###  Notes
+
+* The **CSI NFS driver** offers improved compatibility with newer Kubernetes versions and is the **recommended** provisioner going forward.
+* Avoid reusing the old Helm release metadata (`meta.helm.sh/*`) to prevent installation or upgrade conflicts.
+