Doc/comprehensive improvements

[steininger]

Enhance POS API Documentation with Italian Fiscal Requirements and Implementation Guidelines

• [x] Read the Contributor Guide and Code of Conduct
• [x] Included inline documentation with examples and implementation details
• [x] Added comprehensive documentation for all features and requirements
• [x] Related to issue #[TBD] - Improve POS API documentation with country-specific requirements

Description

This PR significantly enhances the POS System API documentation with comprehensive coverage of Italian fiscal requirements and implementation guidelines. The changes focus on providing clear, practical examples and detailed technical specifications for developers implementing the fiskaltrust.Middleware in the Italian market.

Key Improvements

1. Italian Fiscal Requirements

   • Detailed receipt type definitions with RT signature requirements
   • Complete VAT rates and nature codes reference
   • Payment method specifications with RT requirements
   • Compliance guidelines for daily operations

2. Implementation Examples

   • Lottery receipt integration with customer code handling
   • Split payment scenarios with multiple VAT rates
   • Electronic invoice generation with SDI requirements
   • Daily closing operations with lottery data

3. Technical Implementation

   • RT device state management and monitoring
   • Advanced error recovery patterns
   • Queue management with persistence
   • Data transmission verification

4. Security and Best Practices

   • Token management implementation
   • Request encryption examples
   • Secure credential storage
   • Audit logging patterns

Documentation Structure

• Added clear section headers and navigation
• Included practical code examples
• Provided detailed implementation guidelines
• Added error handling scenarios
• Included security best practices

Testing and Validation

• Verified all code examples for syntax correctness
• Validated JSON structures against schema
• Confirmed RT device state codes
• Verified VAT rates and nature codes

This enhancement aims to provide developers with comprehensive, practical guidance for implementing the POS System API in the Italian market, with a focus on compliance, reliability, and security.

Fixes #[TBD] - Improve POS API documentation with country-specific requirements