Add Navigator.Config Showcase to Example App

We should add a showcase of the Config props (https://github.com/airbnb/native-navigation/blob/master/docs/api/navigator-config.md) to the example app.

I can get started on it this weekend. I just wanted to make sure no one else is already working on it.

Start with docs/api/navigator-config.md to list the Config props, then inspect the example app to find its navigation showcase entry point. Done means the example app visibly demonstrates the documented Config props; verify the existing example behavior after the change.

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