Update README following playbook guidelines

[devin-ai-integration]

Summary

Rewrites the README.md following playbook guidelines to improve documentation quality and structure. Also updates deprecated GitHub Actions versions to fix CI failures.

README changes:

• Added clear one-sentence project description at the top
• Added table of contents for navigation
• Reorganized into logical sections: Overview, Features, Requirements, Quick Start, Project Structure, API Documentation, GraphQL Support, Database, Testing, Code Formatting, Docker, Using with a Frontend, Contributing, License
• Added license badge
• Improved code blocks with proper syntax highlighting
• Added API endpoint reference table
• Added Devin attribution

CI fix:

• Updated actions/checkout, actions/setup-java, and actions/cache from v2 to v4 to resolve deprecated action failures

Review & Testing Checklist for Human

• Verify the project structure diagram in README matches the actual src/main/java/io/spring/ directory layout
• Confirm all external links work (RealWorld repo, Netflix DGS Framework, Spotless, etc.)
• Verify CI passes with the updated GitHub Actions versions

Suggested test plan: Review the rendered README on GitHub to ensure formatting looks correct and all anchor links in the table of contents work properly.

Notes

Requested by: milind@cognition.ai (@milind-cognition)

Devin Session: https://app.devin.ai/sessions/7e79b56412c540448fd01fe08bae2975

[devin-ai-integration]

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