Improve README: simplify, focus on nanoid, remove emojis

Major changes:
- Remove table of contents and emojis (more professional)
- Split installation: minimal nanoid-only at top, sortable at bottom
- Remove LEAKPROOF (Neon.com compatibility)
- Simplify intro and comparison table (focus on nanoid)
- Reorganize: Installation → Quick Start → Usage → Sortable (Advanced)
- Consolidate examples, reduce redundancy
- Standardize performance to 84K IDs/sec
- Fix repo URL placeholder

Result: ~220 lines core content (from 450), 95% focus on nanoid

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: elitan