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 +1,35 @@
+---
+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.
+3) CSI storageClass should be created in which you want to migrate.
+   1) Example: CSI [storageclass](https://github.com/rook/rook/blob/master/cluster/examples/kubernetes/ceph/csi/rbd/storageclass.yaml).
+
+**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:
+   ### To migrate single PVC:
+      1) `<binary_name> --pvc=<pvc-name> --pvc-namespace=<pvc-namespace> --destinationstorageclass=<storageclass-name-to-migrate-in> --rook-namespace=<rook-ceph> --ceph-cluster-namespace=<ceph-cluster-namespace>`
+         1) `--pvc` :**required**: reads name of the pvc to migrate
+         2) `--pvc-namespace` :**required**: reads the namespace of the PVC which is going to migrate
+         3) `--destinationstorageclass` :**required**: read the name of the storageclass in which you want mirgrate.
+         4) `-rook-namespace`: reads namespace where rook operator is running. **Defalult** rook-ceph.
+         5) `--ceph-cluster-namespace`: namespace where ceph cluster is created. **Defalult** rook-ceph.
+   ### To mirage all the pvc in storageclass:
+      1) WIP