Add trigger and audit logging example to PostgreSQL advanced concepts

[DivyanshGarg380]

Description:
This PR enhances the Advanced Concepts in PostgreSQL documentation by adding a comprehensive example demonstrating how to create and use triggers for auditing database changes.

Changes included:

• Target Table Creation
  • Added an example target_table to demonstrate the trigger functionality.
• Audit Table Creation.
  • Added an audit_table that logs all INSERT, UPDATE, and DELETE operations performed on the target table.
  • Includes columns for event_type, event_time, table_name, old_data, and new_data.
• Trigger Function (log_changes)
  • Created a PL/pgSQL trigger function to automatically log changes to the audit_table.
  • Handles all three operations:
    • INSERT: Logs the new record.
    • UPDATE: Logs both old and new record states.
    • DELETE: Logs the deleted record.
  • Uses to_jsonb() to store the row data in JSONB format for flexible querying.
• Trigger Attachment
  • Demonstrates how to attach the log_changes trigger function to the target_table.
  • Trigger executes AFTER INSERT, UPDATE, or DELETE for each affected row.

Benefits:

• Provides a real-world use case of triggers in PostgreSQL.
• Helps readers understand how to maintain an audit trail automatically.
• Complements existing documentation on constraints, joins, and other advanced PostgreSQL features.

Usage:
Once merged, users can follow this example to implement audit logging for any table in their database.