Add playground starter example

### What should we do?

We should add the playground to our [starter examples](https://github.com/bpmn-io/form-js-examples/tree/master/starter).

### Why should we do it?

Offer an easy entry point on how to consume the form-js-playground.

Cf. https://github.com/bpmn-io/form-js/issues/328

Also cf. to [other examples in bpmn-io projects](https://github.com/bpmn-io/bpmn-js-examples)

Start in the starter directory and compare its existing examples with the examples in bpmn-js-examples. Review how form-js-playground is consumed, then add a starter example that provides an easy entry point for using it and verify it follows the repository's existing example structure.

Written by the indexing model from the issue text.