add CI/CD: MkDocs build + Docker deploy on port 20000

- .gitea/workflows/deploy.yaml: Actions pipeline
- Dockerfile: nginx:alpine serving site/
- nginx.conf: SPA routing + gzip + cache
- .dockerignore: exclude source files
- .gitignore: exclude site/ (built in CI)

nginx.conf

@@ -0,0 +1,26 @@
+server {
+    listen 80;
+    server_name _;
+
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # MkDocs Material 使用 SPA 风格路由，需要回退到 index.html
+    location / {
+        try_files $uri $uri/ $uri.html /index.html;
+    }
+
+    # 404 也回退到 index.html（搜索功能需要）
+    error_page 404 /404.html;
+
+    # Gzip 压缩
+    gzip on;
+    gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
+    gzip_min_length 256;
+
+    # 静态资源缓存
+    location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
+        expires 7d;
+        add_header Cache-Control "public, no-transform";
+    }
+}