Cupertino gallery app

[ericwindmill]

Resolves #2519

Part of the larger effort to bring the samples/repo into a stable and maintainable condition that also remains useful. See: #2409

Pre-launch Checklist

• [x] I read the [Flutter Style Guide] recently, and have followed its advice.
• [x] I signed the [CLA].
• [x] I read the [Contributors Guide].
• [x] I have added sample code updates to the [changelog].
• [x] I updated/added relevant documentation (doc comments with ///).

[ericwindmill]

@sethladd @MitchellGoodwin -- I'm adding a Cupertino gallery app to match the Material 3 Demo sample. This is also the sample I want to use to experiment with AI maintenance as Cupertino is updated with releases.

Can you double check the list of widgets that are being shown and let me know if anything important is missing or anything should be removed?

[Piinks]

Greetings from stale PR triage! 👋 Is this change still on your radar?