docs: add instance to sbs migration guide

docs/guides/migrate-instance-volume-to-sbs.md

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+---
+page_title: "Migrating the management of Instance Block Storage volumes to SBS"
+---
+
+# Migration
+
+This page describes how to migrate the management of your Instance's volumes from Block Storage legacy to SBS (Scaleway Block Storage).
+This documentation **only applies if you have created Block Storage legacy volumes** (`b_ssd`).
+
+Migration of local volumes is not supported, you will have to migrate your data manually.
+
+Find out about the advantages of migrating from the Instance API to the Block Storage API for managing block volumes in the [dedicated documentation](https://www.scaleway.com/en/docs/block-storage/reference-content/advantages-migrating-to-sbs/).
+
+## Migrate your implicit root_volume
+
+If your infrastructure includes a server with a root volume that must be migrated:
+
+```terraform
+resource scaleway_instance_server "server" {
+  type  = "PLAY2-PICO"
+  image = "ubuntu_jammy"
+  root_volume {
+    volume_type = "b_ssd"
+  }
+}
+```
+
+In the snippet above, the `root_volume` type is explicitly configured as a `b_ssd`. This configuration must be removed to prepare for migration.
+
+```terraform
+resource scaleway_instance_server "server" {
+  type  = "PLAY2-PICO"
+  image = "ubuntu_jammy"
+}
+```
+
+You can now migrate your root_volume using the [Scaleway CLI documentation](https://www.scaleway.com/en/docs/instances/how-to/migrate-volumes-snapshots-to-sbs/#migrating-an-existing-block-storage-volume-to-scaleway-block-storage-management).
+After migration, the output when running `terraform plan` should be empty as the provider should have picked up that the volume is now managed by the Scaleway Block Storage API.
+
+## Migrate your explicit volumes
+
+If your infrastructure includes servers and explicit volumes.
+
+```terraform
+resource scaleway_instance_volume "root_volume" {
+  size_in_gb = 20
+  type = "b_ssd"
+}
+
+resource scaleway_instance_volume "volume" {
+  size_in_gb = 20
+  type = "b_ssd"
+}
+
+resource scaleway_instance_server "server" {
+  type  = "PLAY2-PICO"
+  root_volume {
+    volume_id = scaleway_instance_volume.root_volume.id
+  }
+
+  additional_volume_ids = [scaleway_instance_volume.volume.id]
+}
+```
+
+You can rely on Terraform to perform the migration which will be done in 2 steps.
+The first one will be a transitional state to migrate and is described in the next snippet.
+In this snippet, the `migrate_to_sbs` field will prevent the old volume state from being updated during the migration.
+
+
+```terraform
+resource scaleway_instance_volume "root_volume" {
+  size_in_gb = 20
+  type = "b_ssd"
+  migrate_to_sbs = true # Mark migration to avoid failure
+}
+
+resource scaleway_block_volume "root_volume" {
+  size_in_gb = 20
+  iops = 5000 # b_ssd is a 5000 iops volume
+  instance_volume_id = scaleway_instance_volume.root_volume.id # block resource will handle migration
+}
+
+resource scaleway_instance_volume "volume" {
+  size_in_gb = 20
+  type = "b_ssd"
+  migrate_to_sbs = true # Mark migration to avoid failure
+}
+
+resource scaleway_block_volume "volume" {
+  size_in_gb = 20
+  iops = 5000 # b_ssd is a 5000 iops volume
+  instance_volume_id = scaleway_instance_volume.volume.id # block resource will handle migration
+}
+
+
+resource scaleway_instance_server "server" {
+  type  = "PLAY2-PICO"
+  root_volume {
+    volume_id = scaleway_block_volume.root_volume.id # Start using your new resource
+  }
+
+  additional_volume_ids = [scaleway_block_volume.volume.id] # Start using your new resource
+}
+```
+
+The first migration step should have created your new Block Storage volumes and updated the existing instance_volume resources.
+After confirming the migration is successful, you must remove the old Instance's resources manually.
+Terraform's scaleway_instance_volume resource cannot delete a volume that has been migrated to the Scaleway Block Storage API. Before applying the final step, check in the Scaleway [console](https://console.scaleway.com) or using the [CLI](https://cli.scaleway.com/block/#list-volumes) to confirm that your volume was successfully migrated.
+
+
+```terraform
+resource scaleway_block_volume "root_volume" {
+  size_in_gb = 20
+  iops = 5000
+}
+
+resource scaleway_block_volume "volume" {
+  size_in_gb = 20
+  iops = 5000
+}
+
+resource scaleway_instance_server "server" {
+  type  = "PLAY2-PICO"
+  root_volume {
+    volume_id = scaleway_block_volume.root_volume.id
+  }
+
+  additional_volume_ids = [scaleway_block_volume.volume.id]
+}
+```