[Docs] Playground for app examples and demos

[castastrophe]

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Description of issue

Really cool interactive example: https://lit.dev/tutorial/

Could make use of the playground components: https://github.com/PolymerLabs/playground-elements

[castastrophe]

Also found this example: https://start.vaadin.com/welcome

[Westbrook]

Combination of the playground-elements with MD for reference: https://github.com/apollo-elements/apollo-elements/blob/main/docs/index.md

[Westbrook]

These designs might be an interesting inclusion as "demo apps" when made interactive: https://spectrum.corp.adobe.com/page/tutorials/ While also being useful learning content, generally.

[hunterloftis]

This is another good candidate for post- #1663 enhancements