Improve README readability and project overview

[keemgdeok]

Summary

• add concise Release, CI, and Java badges plus a clearer project value statement
• keep Quick Start prominent and add a compact metadata flow diagram
• surface rule behavior and the 100k-file benchmark in scannable tables
• simplify release status, demo wording, and benchmark terminology
• use the downloaded release JAR consistently in REST catalog examples

Verification

• Spotless check passed
• GitHub Markdown tables and Mermaid rendering verified
• badge and documentation links verified
• GitHub Actions build passed

Documentation only; no runtime behavior changed.