Update .env.example to replace PostgreSQL configuration with DATABASE_URL for improved connection management

Author: agtk-nachov

RAGManager/.env.example

@@ -1,9 +1,4 @@
-# PostgreSQL Configuration
-POSTGRES_USER=postgres
-POSTGRES_PASSWORD=postgres
-POSTGRES_DB=vectordb
-POSTGRES_HOST=localhost
-POSTGRES_PORT=5432
+DATABASE_URL=postgresql://postgres:postgres@postgres:5432/vectordb
 
 # RabbitMQ Configuration
 RABBITMQ_USER=guest