docs: create a Lingo.dev CLI integration guide for Flutter

[verma-divyanshu-git]

What

This PR adds a comprehensive Flutter CLI integration guide following the structure and content of existing framework guides.

Why

Closes #1340

Flutter developers need a step-by-step guide to integrate Lingo.dev CLI with their Flutter applications using ARB (Application Resource Bundle) files.

How

The guide covers:

• Setting up a Flutter project with localization dependencies
• Creating and configuring ARB files
• Configuring Lingo.dev CLI with the flutter bucket type
• Implementing locale switching in Dart
• Testing translations with flutter run

Testing

• Tested the complete guide by following all steps

[davidturnbull]

Hi 👋

Thanks for the submission. I'll follow this guide shortly and let you know if I have any feedback.

😄

[sumitsaurabh927]

@verma-divyanshu-git there are failing checks, please resolve.

[verma-divyanshu-git]

all checks passing @sumitsaurabh927 . Can you approve and merge