Add .env.example template with environment variables documentation

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

.gitignore

@@ -16,6 +16,7 @@ _.log
 report.[0-9]_.[0-9]_.[0-9]_.[0-9]_.json
 
 # dotenv environment variable files
+# Only .env.example is allowed to be committed
 .env
 .env.development.local
 .env.test.local