Enhance concert management API tutorial with detailed descriptions and examples

- Updated the introduction to clarify the tutorial's focus on building a concert management REST API using Goa.
- Improved API design documentation with structured sections for API definition, service structure, and key features.
- Enhanced method descriptions for CRUD operations, including detailed payload requirements and error handling.
- Added OpenAPI integration details and examples for better documentation.
- Updated implementation details for the service, including in-memory storage and UUID generation.
- Improved testing instructions and error handling examples for better user experience.

Author: raphael