Add example for Callable Workflows

We should add a case to the example section: Using the Cypress Action in a Callable Workflow.

Callable workflows are reusable stand-alone .yaml files that can be called by any caller workflow. This allows you to use the same configuration for multiple workflows, for example by adding the callable workflow into the `on: pull_request` events for PR as well as nightflow validation workflows.

Callable workflows are powerful and extensible, but also somewhat tricky to use. Which makes the topic a good candidate for an example.

If we go forward, I'm happy to write out the documentation for ths.