Create a documentation article on creating a basic app

[stackotter]

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It'd be great to have a full start-to-finish tutorial for making a basic app of some sort with SwiftUI. Modelling it off SwiftUI's landmarks app tutorial could be a good starting point. To create a new DocC article just copy Basic Usage.md and modify it as necessary. Preferably the tutorial should also include images, but it's fine if it doesn't at first. If photos are included, ensure that they are compressed before committing them to avoid greatly increasing the size of the SwiftCrossUI repository.