docs: add top level file guide to flutter_weather

[EasonLin0716]

Status

READY

Breaking Changes

NO

Description

There's only adding models.dart in here https://bloclibrary.dev/tutorials/flutter-weather/#barrel-file-2

I think it would be better if also guide users to add lib/weather/weather.dart with content

export './models/models.dart';

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore