docs: add Nginx reverse proxy configuration with SSL guide

- Add nginx-example.conf with complete SSL configuration
- Add SSL-SETUP.md with step-by-step setup guide
- Include Let's Encrypt certbot instructions
- Add WebSocket support for real-time collaboration
- Include security headers and best practices
- Add troubleshooting section

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

nginx-example.conf

@@ -0,0 +1,83 @@
+# Nginx 反向代理配置範例 (含 SSL)
+# 放置位置：/etc/nginx/sites-available/codimd
+
+# HTTP - 自動重定向到 HTTPS
+server {
+    listen 80;
+    listen [::]:80;
+    server_name your-domain.com;
+
+    # Let's Encrypt 驗證目錄
+    location /.well-known/acme-challenge/ {
+        root /var/www/certbot;
+    }
+
+    # 其他 HTTP 流量重定向到 HTTPS
+    location / {
+        return 301 https://$server_name$request_uri;
+    }
+}
+
+# HTTPS - 主要服務
+server {
+    listen 443 ssl http2;
+    listen [::]:443 ssl http2;
+    server_name your-domain.com;
+
+    # SSL 證書配置 (Let's Encrypt)
+    ssl_certificate /etc/letsencrypt/live/your-domain.com/fullchain.pem;
+    ssl_certificate_key /etc/letsencrypt/live/your-domain.com/privkey.pem;
+
+    # SSL 安全配置
+    ssl_protocols TLSv1.2 TLSv1.3;
+    ssl_ciphers 'ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384';
+    ssl_prefer_server_ciphers off;
+    ssl_session_cache shared:SSL:10m;
+    ssl_session_timeout 10m;
+    ssl_session_tickets off;
+
+    # HSTS (HTTP Strict Transport Security)
+    add_header Strict-Transport-Security "max-age=63072000; includeSubDomains; preload" always;
+
+    # 其他安全標頭
+    add_header X-Frame-Options "SAMEORIGIN" always;
+    add_header X-Content-Type-Options "nosniff" always;
+    add_header X-XSS-Protection "1; mode=block" always;
+    add_header Referrer-Policy "no-referrer-when-downgrade" always;
+
+    # 用戶端上傳大小限制
+    client_max_body_size 50M;
+
+    # 反向代理到 CodiMD
+    location / {
+        proxy_pass http://127.0.0.1:3000;
+        proxy_http_version 1.1;
+
+        # WebSocket 支援 (CodiMD 即時協作需要)
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "upgrade";
+
+        # 標準代理標頭
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_set_header X-Forwarded-Host $host;
+        proxy_set_header X-Forwarded-Port $server_port;
+
+        # 超時設定
+        proxy_connect_timeout 60s;
+        proxy_send_timeout 60s;
+        proxy_read_timeout 60s;
+
+        # 緩衝設定
+        proxy_buffering off;
+        proxy_request_buffering off;
+    }
+
+    # 健康檢查端點 (可選)
+    location /health {
+        access_log off;
+        proxy_pass http://127.0.0.1:3000;
+    }
+}