docs: revamp README with comprehensive example usage and guidance

[appleboy]

• Rewrite and expand the README to provide a detailed overview of the example categories, launch options, and usage instructions
• Add an extensive table of contents with clear links to all major example types
• Document step-by-step instructions for running examples in various modes (console, restapi, a2a, webui)
• Clarify environment variable requirements and configuration patterns across examples
• Distinguish between full and production launchers, specifying their supported modes
• Add sections on contributing, common patterns, and integration prerequisites for advanced examples
• Improve consistency, clarity, and completeness throughout the guide

[gemini-code-assist[bot]]

Summary of Changes

Hello @appleboy, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the documentation for the ADK Go examples by revamping the examples/README.md. The update aims to provide a much clearer and more comprehensive guide, making it easier for developers to understand, run, and contribute to the various ADK Go examples. This will improve the onboarding experience and overall usability of the example suite.

Highlights

• Comprehensive README Revamp: The examples/README.md has been completely rewritten and expanded to offer a detailed guide for all ADK Go examples, significantly improving its clarity and completeness.
• Enhanced Navigation & Usage: A new Table of Contents, detailed 'Getting Started' instructions, and categorized examples (Basic, Integration, Tool, Workflow Agent, Advanced) have been added to improve discoverability and ease of use.
• Clear Launcher System Explanation: The documentation now clearly distinguishes between full.NewLauncher() and prod.NewLauncher(), outlining their supported modes (console, restapi, a2a, webui) and use cases.
• New Guidance Sections: New sections have been introduced covering common patterns (environment variables, configuration), contributing guidelines for new examples, and additional resources for further learning.

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[appleboy]

@dpasiukevich Any feedback?