docs: refresh README cover image and remove stale screenshots

[oliverlaz]

🎯 Goal

Refresh the SDK's README cover image and clean up the long-unused screenshots/ directory.

🛠 Implementation details

• Added new cover at .readme-assets/stream-chat-react-native-cover.png.
• Updated README.md and examples/SampleApp/README.md to reference the new asset via relative paths (also improved alt text).
• Deleted the entire screenshots/ directory (~16 MB, last touched in 2022). References to it from package/CHANGELOG.md are historical entries with already-broken relative paths and are intentionally left untouched.

🎨 UI Changes

No app-side UI changes — README cover image only.

🧪 Testing

• Verified the new image renders in both READMEs locally.
• Confirmed no remaining non-historical references to screenshots/.

☑️ Checklist

• I have signed the Stream CLA (required)
• PR targets the develop branch
• Documentation is updated
• New code is tested in main example apps, including all possible scenarios
  • SampleApp iOS and Android
  • Expo iOS and Android