cmd/config: Add examples and tutorials for config functions

- Add examples under `functions`
- Add built-in tutorial for functions

functions/examples/template-heredoc-cockroachdb/local-resource/example-use.yaml

@@ -0,0 +1,13 @@
+# Copyright 2019 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+# call `kustomize config run-fns` on a directory containing this file
+apiVersion: examples.config.kubernetes.io/v1beta1
+kind: CockroachDB
+metadata:
+  name: demo
+  configFn:
+    container:
+      image: gcr.io/kustomize-functions/example-cockroachdb:v0.1.0
+spec:
+  replicas: 3