Add comprehensive LLM documentation for forms library

- Add llms.txt with complete implementation guide for LLMs
- Covers all form components: TextField, Textarea, Checkbox, Switch, RadioGroup, DatePicker, DropdownMenuSelect, OTPInput
- Includes Zod validation patterns and server-side handling
- Documents component customization system and advanced patterns
- Provides comprehensive example demonstrating all features
- Designed specifically for LLM consumption and implementation

Author: codegen-sh[bot]