Enhance ElevenLabs README with comprehensive usage guide
Added a detailed "How to Use This Cookbook" section that guides users through:
- Step 1: Environment setup with API keys and dependencies
- Step 2: Working through the notebook to learn concepts
- Step 3: Running the production script for hands-on experience
Also expanded the "More About ElevenLabs" section with additional resources including Voice Library, API Playground, and SDK links.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>