docs: add guide doc for flex migration

Adding guide doc for flex migration steps.

Signed-off-by: subhamkrai <srai@redhat.com>

Documentation/flexMigration/flex-to-csi-migration.md

@@ -0,0 +1,31 @@
+---
+title: Migration
+weight: 4100
+indent: true
+---
+
+# Flex to CSI Migration
+
+The tool [persistent-volume-migrator](https://github.com/ceph/persistent-volume-migrator) aims to help migrate Flex rbd Volumes to Ceph-Csi.
+
+## Prerquisite
+
+This guide assumes you have created a Rook cluster with Flex
+1) Minimum ceph version should be Ceph-CSI supported.
+2) Volume should be in BOUND state and should **not** be in use.
+
+**NOTE**: The Migration procedure will come with a downtime; As we need to scale down the applications using the Volumes before migration.
+
+## Steps
+1) Create rbac required to run migrator pod
+   1) `kubectl create -f cluster/examples/kubernetes/ceph/flex-migration/rbac.yaml`
+2) Create migrator pod
+   1) `kubectl create -f cluster/examples/kubernetes/ceph/flex-migration/migrator.yaml`
+3) Run command to start migration:
+   1) To migrate single PVC:
+      1) `<binary_name> --pvc=rbd-pvc --pvc-namespace=default --destinationstorageclass=csi-rook-ceph-block`
+         1) `--pvc` reads name of the pvc to migrate
+         2) `--pvc-namespace` reads the namespace of the PVC which is going to migrate
+         3) `--destinationstorageclass` read the name of the storageclass in which you want mirgrate.
+   2) To mirage all the pvc in storageclass:
+      1) WIP