docs: polish README with accurate versions and structure

[kiro-agent]

This pull request was generated by @kiro-agent 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
_{Learn about Kiro autonomous agent}

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Summary

Rewrites the README.md to be accurate, well-structured, and visually polished.

Changes

• Fixed version numbers to match actual source code: Java 25, GraalVM CE 25.0.2, Maven 3.9.14, native-maven-plugin 0.11.5 (the previous README referenced Java 17, GraalVM 17.0.9, Maven 3.9.6, native-maven-plugin 0.9.28)
• Updated badges to reflect the correct Java 25 and GraalVM 25.0.2 versions, and added a License badge
• Added a Table of Contents for easy navigation across sections
• Added a comparison table in the "Why AWS CRT Client?" section explaining the benefits over the default Netty NIO and Apache HTTP clients
• Reformatted Prerequisites as a table with links to each tool
• Reformatted deployed resources as a table for quick scanning
• Added Contributing and License sections linking to CONTRIBUTING.md and the MIT No Attribution license
• Added section dividers (---) between major sections for visual clarity
• Preserved the architecture ASCII diagram, CloudWatch Logs Insights query, project structure tree, and result.png reference