add "document-start" 1st line to all yaml files

The YAML Specification requires three daashes `---` as marker for the
start of a content block. That means, each YAML file needs to start with
`---` on the first line (or after initial comments).

See-also: https://yaml.org/

examples/kubernetes/invalid-snapshot-v1.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: snapshot.storage.k8s.io/v1
 kind: VolumeSnapshot
 metadata:

examples/kubernetes/pvc.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:

examples/kubernetes/restore.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: v1
 kind: PersistentVolumeClaim
 metadata:

examples/kubernetes/snapshot-v1.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: snapshot.storage.k8s.io/v1
 kind: VolumeSnapshot
 metadata:

examples/kubernetes/snapshotclass-v1.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: snapshot.storage.k8s.io/v1
 kind: VolumeSnapshotClass
 metadata:

examples/kubernetes/storageclass.yaml

@@ -1,3 +1,4 @@
+---
 apiVersion: storage.k8s.io/v1
 kind: StorageClass
 metadata: