feat: expand INDEX.md as comprehensive navigation guide (#657)

- Rewrite INDEX.md as a full documentation index mapping all 45+ markdown files
- Organize into sections: Quick Start, Architecture & Design (incl. ADRs), Smart Contract, Backend, Frontend, Security, Operations, Feature Docs, GitHub Templates, Changelog
- Add one-line description for every file with relative links
- Link INDEX.md from README.md via a prominent callout at the top of the architecture section

Author: northvictor