docs: Update README.md to not refer to the Flutter SDK for Firebase as FlutterFire anymore

[puf]

Description

Update to not refer to the Flutter SDK for Firebase as FlutterFire anymore, and to link to the official documentation.

Type of Change

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

[CLAassistant]

All committers have signed the CLA.