docs(router-proxy): 更新环境变量配置说明

- 在 .env.example 中添加了关于 REGISTRY_PATH 和 OUTPUT_PATH 的注释说明，
  解释这两个变量通常不需要手动配置，脚本会自动基于相对路径推导
- 在 README.md 中补充了相关配置项的中文说明，强调默认值比绝对路径更稳定

router-proxy/.env.example

@@ -5,5 +5,9 @@ PROXY_CONTAINER_NAME=nano-router-proxy
 PROXY_NETWORK_NAME=nano-instance-edge
 PROXY_HTTP_PORT=8088
 
-REGISTRY_PATH=/home/ivan/xuan/nano_project/app-instance/runtime/registry/instances.json
-OUTPUT_PATH=/home/ivan/xuan/nano_project/router-proxy/runtime/conf.d/instances.conf
+# Optional host-side overrides.
+# In most cases leave these unset and use start-proxy.sh defaults,
+# which resolve paths relative to the script location.
+#
+# REGISTRY_PATH=../app-instance/runtime/registry/instances.json
+# OUTPUT_PATH=./runtime/conf.d/instances.conf