Sort documentation into topics

[stackotter]

By default DocC sorts definition into a few topics — classes, structs, protocols, etc — which aren't all that useful to users. Luckily, DocC allows documentation to be sorted into custom topics in a pretty simple manner; you just need to edit SwiftCrossUI.md.

Proposed topics

Feel free to use your own judgement, but here's one way we could split the docs into topics,

• Creating apps: App, SceneBuilder, ViewBuilder
• Views: View, Button, Text, etc.
• Scenes: Scene, WindowGroup, etc.
• State: Observable, Publisher, etc.
• Implementation details: ViewGraph, ViewGraphNode, SceneGraphNode, ForEachChildren, WindowGroupNode, etc (basically just everything else)

Give each topic a short helpful description so that people can understand what each one is about. Implementation details are intentionally accessible to make SwiftCrossUI more accessible, but are only intended for advanced users.

How to create topics

Here's how you'd create a Views topic containing all view types,

SwiftCrossUI.md

...

## Topics

...

### Views

SwiftCrossUI has a multitude of built in views that you can combine to create complex UIs.

- ``Button``
- ``Text``
- ``HStack``
- ``VStack``
- ``ForEach``
- ...

[NinjaCheetah]

Should be resolved by #70.