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/

deploy/kubernetes/csi-snapshotter/rbac-external-provisioner.yaml

@@ -8,6 +8,7 @@
 #   provisioner, in which case leadership election must be enabled;
 #   this influences the RBAC setup, see below
 
+---
 apiVersion: v1
 kind: ServiceAccount
 metadata: