docs: update .env.example with detailed instructions and configuration notes

.env.example

@@ -1,5 +1,21 @@
-# Backend API Base URL
+# ENVIRONMENT CONFIGURATION TEMPLATE
+#
+# This file serves as a template for environment variables.
+# DO NOT include any sensitive data (passwords, API keys, etc.) in this file.
+#
+# INSTRUCTIONS
+# 1. Copy this file to `.env`, `.env.development`, or `.env.production` depending on your
+#    target environment.
+# 2. Replace the placeholder values with your actual configuration.
+# 3. Alternatively, ensure these variables are defined within your system's environment
+#    settings before running the application.
+
+# The base URL fot the backend API service.
+# Ensure this matches your Spring Boot server address (e.g., http://localhost:8080).
 VITE_API_BASE_URL=/api
 
-# Redux persistent storage location, use `local` for local storage and `session` for session storage
-VITE_REDUX_STORAGE=local
+# Determines where Redux state is persisted on the client side.
+# Available options:
+# - `local`:   Persists data in LocalStorage (remains after closing the browser).
+# - `session`: Persists data in SessionStorage (cleared when the tab is closed).
+VITE_REDUX_STORAGE=local