docs: restructure CONTRIBUTING.md with detailed acceptance criteria

Expand contributing guidelines with:
- Quick checklist for submissions
- Three-tier acceptance criteria (Industry Standard, Rising Star, Hidden Gem)
- Quality requirements and automatic rejection rules
- Entry format reference with examples
- PR description template

This provides clear, actionable guidance for contributors and reduces ambiguity in the review process.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>