docs: add contributor guide

[leoncheng57]

Summary

• add canonical contributor setup, architecture, conventions, verification, PR, screenshot, and security guidance
• add a self-contained visual contributor reading index with file-safe links, responsive light/dark themes, accessible controls, and print support
• link both contributor entry points from the project README

Files

• CONTRIBUTING.md
• docs/contributing/index.html
• README.md

Verification

• validated every documented npm run command against package.json
• checked repository-relative targets across 24 Markdown files and the HTML index
• confirmed new documentation is ASCII-only and git diff --check passes
• visually checked the index from file:// at desktop and 390x740 mobile widths, including light/dark controls and horizontal overflow
• npm run typecheck
• npm test (231 passed)
• npm run build
• CI=1 npm run test:e2e (138 passed, 1 expected screenshot-runner skip)