docs(sqlite-ddl): add DDL Schema diagram for database structure reference

What
- Added `DDL Schema.png` illustrating SQLite Data Definition Language (DDL).
- Diagram shows structure and relationships of tables used in the database.
- Covers key schema operations:
  - `CREATE TABLE` → defining tables with columns and constraints.
  - `ALTER TABLE` → modifying schema structure.
  - `DROP TABLE` → deleting tables.
- Provides visual representation to complement SQL examples in code.

Why
- Helps developers understand schema-level operations in SQLite.
- Acts as a quick reference for database design and schema management.
- Bridges gap between abstract SQL commands and real database structure.
- Enhances learning by combining code (`Database.java`) with visual aids.

How
- Diagram highlights key elements of DDL:
  - Table definitions with column types.
  - Constraints like `PRIMARY KEY`, `NOT NULL`, `UNIQUE`.
  - Relationships (if any) for schema evolution.
- Positioned as part of JDBC + SQLite learning resources.

Key Notes
- DDL commands affect database structure, not the actual data.
- Misuse of `DROP` or `ALTER` can cause data/schema loss → use cautiously.
- Use version control (migration scripts) for schema changes in real projects.

Real-life Applications
- Designing and maintaining application schemas.
- Creating normalized tables for relational integrity.
- Supporting schema migrations in production systems.
- Teaching database fundamentals alongside JDBC.

Future Improvements
- Add ER diagrams for complex relationships.
- Provide SQL migration scripts matching the schema diagram.
- Annotate diagram with example queries for better context.
- Extend documentation to include DML (Data Manipulation Language).

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan