cmd/config: Add examples and tutorials for config functions

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

functions/examples/template-heredoc-cockroachdb/image/Dockerfile

@@ -0,0 +1,12 @@
+# Copyright 2019 The Kubernetes Authors.
+# SPDX-License-Identifier: Apache-2.0
+
+FROM golang:1.13-stretch
+ENV CGO_ENABLED=0
+RUN go get -v sigs.k8s.io/kustomize/kustomize
+
+FROM alpine:latest
+RUN apk add --no-cache bash
+COPY --from=0 /go/bin/kustomize /usr/local/bin
+COPY cockroachdb-template.sh /usr/local/bin/config-function
+CMD ["config-function"]