Add .env-example file and update configuration for environment variables

- Created .env-example for environment variable setup
- Updated README to reflect changes in .env file creation
- Modified config.py to include new environment variables and DATABASE_URL property
- Enhanced database.py with connect and disconnect functions
- Updated main.py to manage database connection lifecycle
- Adjusted compose.yaml for consistent environment variable usage

.env-example

@@ -0,0 +1,5 @@
+postgres_user=exampleUser
+postgres_password=examplePasswd
+postgres_db=exampleDB
+db_port=5432
+api_port=8000