Add comprehensive documentation and doctests to core modules

### Added
- **📖 Comprehensive Module Documentation**
  - Added detailed module-level documentation for lib.rs, job.rs, priority.rs, and worker.rs
  - Included usage examples, feature descriptions, and core concepts explanations
  - Complete documentation of job lifecycle, priority system, and worker configuration

- **📝 Extensive API Documentation**
  - Added doctests with practical examples for all major types and methods
  - Job creation examples with priority, timeout, and scheduling configurations
  - Worker configuration examples with rate limiting, statistics, and priority weights
  - Priority system examples showing weighted vs strict scheduling

- **💻 Interactive Code Examples**
  - Quick start guide with complete working examples
  - Step-by-step job handler creation and worker pool setup
  - Database initialization and job enqueuing examples
  - Error handling and retry configuration demonstrations

### Enhanced
- docs.rs will now display rich documentation with interactive examples
- Users can easily understand the codebase through comprehensive examples
- All major features are documented with practical use cases
- Code examples demonstrate best practices and common patterns

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: CodingAnarchy