feature: add courses by Rivaan Ranawat on Flutter in free-courses-en.md

[HRIDYANSHU054]

What does this PR do?

Add resource(s)

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For resources

Description

Adding three comprehensive Flutter and Dart development video courses/tutorials by Rivaan Ranawat to the relevant section. These resources cover the full spectrum of mobile development using Flutter.

1. Flutter Bloc Tutorial For Beginners | BLoC Architecture/Pattern & State Management Explained
2. Flutter Clean Architecture Full Course For Beginners - Bloc, Supabase, Hive, GetIt
3. The Complete Dart & Flutter Developer Course | Full Tutorial For Beginners to Advanced

Why is this valuable (or not)?

These are high-quality, long-form tutorials that cover essential topics for building production-ready Flutter applications, including State Management and Clean Architecture.

How do we know it's really free?

All of these resources are freely available YT videos.

For book lists, is it a book? For course lists, is it a course? etc.

These are free online video courses/tutorials available on YouTube.

Checklist:

• Search for duplicates.
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction).
• Used an informative name for this pull request.

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!