docs: Add Claude skill for md-book documentation generator

Comprehensive skill documenting:
- CLI usage and all options
- Configuration (book.toml, env vars, priority)
- Two methods for index page creation (custom vs auto-generated cards)
- Features: syntax highlighting, Pagefind search, live reload
- Template customization with Tera
- Deployment guides for multiple platforms
- Testing and feature flags
- Security considerations and troubleshooting

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: AlexMikhalev