flutter: remove useless examples and add release APK (#4853)

pages/common/flutter.md

@@ -3,23 +3,11 @@
 > Google's free, open source, and cross-platform mobile app SDK.
 > More information: <https://github.com/flutter/flutter/wiki/The-flutter-tool>.
 
-- Check the Flutter version:
-
-`flutter --version`
-
-- Display general help:
-
-`flutter help`
-
 - Display help about a specific command:
 
 `flutter help {{command}}`
 
-- Execute a Flutter command:
+- Check if all external tools are correctly installed:
-
-`flutter {{command}}`
-
-- Show information about the installed tooling:
 
 `flutter doctor`
 
@@ -31,6 +19,14 @@
 
 `flutter run -d all`
 
+- Download all packages specified in `pubspec.yaml`:
+
+`flutter pub get`
+
 - Run tests in a terminal from the root of the project:
 
 `flutter test {{test/example_test.dart}}`
+
+- Build a release APK targeting most modern smartphones:
+
+`flutter build apk --target-platform {{android-arm}} {{android-arm64}}`