Improve minimal_subscriber README with overview, message flow explanation, and usage guidance

[yadnyeshwar]

Description

This PR improves the documentation for the minimal_subscriber examples in the rclcpp topics directory.

The goal is to make the examples easier to understand for developers who are new to ROS 2 by providing clearer explanations of the subscriber patterns used in the examples.

Changes made

• Added an Overview section explaining the purpose of the examples
• Introduced a message flow diagram showing the publisher → topic → subscriber → callback structure
• Clarified the behavior of each example implementation
• Added a "When to Use Each Example" section explaining the practical use cases for different subscriber patterns
• Highlighted the recommended class-based node approach (member_function.cpp) commonly used in real ROS 2 applications

These additions aim to improve readability and help developers understand the design trade-offs between different subscriber implementations.

Motivation

The original README listed the examples but did not clearly explain when or why each implementation pattern might be used. This update provides more context and guidance for learners exploring ROS 2 communication patterns.

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Is this user-facing behavior change?

No. This change only improves documentation and does not modify any runtime behavior.

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Did you use Generative AI?

Yes. Generative AI tools were used to help refine the documentation text. All changes were reviewed and validated manually to ensure correctness with ROS 2 concepts.

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Additional Information

This change does not modify any source code and is limited to improving documentation clarity for the minimal_subscriber examples.