docs: add environment setup instructions

- Add .env.example file with template for secure configuration
- Update Quick Start section with detailed environment setup
- Provide both .env file and docker-compose variable methods
- Add security notes and password generation commands
- Document default database credentials with security warning
- Include instructions for generating secure JWT secrets and passwords

.env.example

@@ -0,0 +1,8 @@
+# Environment Variables
+# Copy this file to .env and update the values
+
+# JWT Secret for authentication (generate a secure random string)
+JWT_SECRET=your-secure-jwt-secret-key-here
+
+# PostgreSQL database password
+DBPASS=your-secure-database-password-here