Create example: Templating in a multiline string

## Why
A common use case for Kubernetes ConfigMaps and Secrets are to put values in a multiline string, which may need to be templated. Templating that multiline string isn’t obvious (it requires `yaml.encode()`).
## What
[There exists](https://github.com/vmware-tanzu/carvel-ytt/blob/develop/examples/k8s-config-map-files/config.yml) an example of this. Put this or another example into the official docs.
## How
Either in the playground or in the carvel.dev/ytt docs.

[Here](https://kubernetes.slack.com/archives/CH8KCCKA5/p1647979081840839) is a request for this

Start with the existing examples/k8s-config-map-files/config.yml example linked in the issue and review the official carvel.dev/ytt documentation or playground entry points. Add this example, or an equivalent one, so users can see how to template a multiline ConfigMap or Secret value; completion is the example appearing in the official docs or playground.

Written by the indexing model from the issue text.