docs: comprehensive llms.txt update with FormError documentation

- Add complete FormError component usage patterns and examples
- Include error hierarchy guidelines (field-level vs form-level)
- Document server action patterns for form-level error handling
- Add comprehensive form examples with mixed error scenarios
- Include FormError placement strategies and best practices
- Document custom styling and component override patterns
- Provide complete implementation guide for LLMs
- Merge existing form component documentation with new FormError features
- Add practical examples for authentication, registration, and complex forms

Author: codegen-sh[bot]