Add documentation for Thing Model module and GenericThingProvider

[hendrickhere]

Documentation Enhancement PR - Thing Model Documentation

Addressed Issue

• Limited documentation in the Thing Model module which is crucial for OpenHAB's device management
• Lack of comprehensive documentation in GenericThingProvider.xtend for key functionality
• Missing explanation of core components and their relationships

What I Have Reengineered

• Created detailed README.md documenting the Thing Model architecture
• Added comprehensive class-level documentation to GenericThingProvider
• Enhanced method-level documentation for key functions including:
  • createThingsFromModel
  • createThing
  • createChannels
  • lazyRetryRunnable
• Documented key components and their purposes

Reengineering Strategy

• Analyzed existing codebase to identify documentation gaps
• Focused on critical components (Thing Model and GenericThingProvider)
• Used clear and consistent documentation style following OpenHAB standards
• Added practical examples including Thing definition examples
• Structured documentation hierarchically for better readability
• Ensured documentation follows Xtend conventions

Impact of Changes

• Easier onboarding for new developers to OpenHAB Thing system
• Better understanding of Thing Model architecture and components
• Clearer documentation of GenericThingProvider's role and functionality
• Improved code maintainability through better documentation
• Enhanced readability of complex components