docs: improve README structure and onboarding

[Gingiris]

What changed

• reorganized the README for faster first-time understanding
• added a clear project overview, TOC, and structure section
• added or clarified install / usage / contribution guidance based on the current repo layout

Why

The repository is active, but the current README makes onboarding harder than it needs to be. This PR focuses on structure and first-run clarity without changing product behavior.

README optimized with Gingiris README Generator

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