docs: add comprehensive VPS deployment guide

Covers system requirements, port planning, firewall rules, Docker
installation, environment configuration, DNS setup, production launch
with Traefik/HTTPS, SMTP configuration, backup/restore, upgrades,
and common troubleshooting scenarios.

Linked from both README.md and README.zh-CN.md.

Author: GALIAIS

README.md

@@ -9,7 +9,7 @@
 </p>
 
 <p align="center">
-  <a href="./README.zh-CN.md">简体中文</a>
+  <a href="./README.zh-CN.md">简体中文</a> · <a href="./docs/deployment-guide.md">部署指南</a>
 </p>
 
 ## Overview

README.zh-CN.md

@@ -9,7 +9,7 @@
 </p>
 
 <p align="center">
-  <a href="./README.md">English</a>
+  <a href="./README.md">English</a> · <a href="./docs/deployment-guide.md">部署指南</a>
 </p>
 
 ## 项目简介