Add an example (or real demo project?) showcasing *actual* multiplatform

[bitspittle]

It would be nice to have a demo project that shows a project which targets both HTML and a totally different target, say Desktop or Android.

In that setup, business logic should live inside a common module, while UI should be different for Kobweb (using Compose HTML) and other Compose Web targets.

In the future, CLOVIS's Decouple project may be a champion for this, because then you can actually have a declarative layer that sits on top of everything, in a way that might actually allow unifying the UI layer to some extent.