[Docs] Decouple Screen class from State class in the docs example

[mck182]

I'd like to propose decoupling the Screen from State in the intro docs section to suggest a better pattern.

The Screen is kind of an API that should be exposed, allowing other code to navigate to it, but the State is internal to the Presenter and the Ui, so this updates the docs to not nest the State under the Screen as they are independent.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @mck182 to sign the Salesforce Inc. Contributor License Agreement.