docs: create polished comprehensive README

[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

Replaces the minimal README with a comprehensive, polished project README for the Unicorn Store application.

What's included

• Badges for Java 17, Spring Boot 3.1.4, AWS Lambda, AWS CDK v2, and License
• Architecture diagram with ASCII art showing the request flow from client through API Gateway, Lambda, and EventBridge
• Detailed request flow explaining how a request travels through the system
• Key optimizations section covering SnapStart, async init, AWS CRT HTTP client, and no embedded Tomcat
• Full project structure tree with descriptions for every file
• Tech stack table listing all technologies organized by layer
• Prerequisites section with required tooling
• Step-by-step instructions for build, CDK bootstrap, deploy, and test
• API reference with complete documentation of the POST /unicorns endpoint (request/response examples, field descriptions, error responses)
• Load testing instructions for the included Artillery config
• Screenshots referencing the existing img/result.png and img/logs.png
• Lambda configuration table with all runtime settings
• Cleanup instructions for tearing down the stack
• Resources section with links to relevant AWS documentation and workshops