docs: add comprehensive JDBC README

Added a professional README for the complete Java Database Connectivity (JDBC) section.

The README documents all 17 JDBC topics and provides structured navigation
through the complete database connectivity learning path.

Included:
- JDBC overview and learning objectives
- Complete 17-topic JDBC roadmap
- Topic-wise documentation overview
- Five learning formats: ONEPAGE, DEEPDIVE, TEACHME, 3LEVEL, and DOUBTKILLER
- Complete JDBC folder structure
- JDBC application flow
- Core JDBC components
- JDBC security and SQL injection prevention
- JDBC performance concepts
- Transaction management overview
- Batch processing overview
- JDBC metadata concepts
- Advanced JDBC concepts
- JDBC best practices
- Recommended learning and revision strategies
- Practical JDBC learning approach
- Skills developed through JDBC
- Documentation coverage statistics
- Topic completion checklist
- Overall JDBC completion status
- Repository navigation
- Final JDBC learning takeaway

Documentation coverage:
17 Topics × 5 Learning Formats = 85 Markdown Documents

This README serves as the professional entry point for navigating and understanding the complete JDBC section of the Java learning repository.