Feat: Add HTTPS setup instructions and configuration for Nginx (#8401)

### What problem does this PR solve?

_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change: Documentation Update/Refactoring

#### Summary
Adds HTTPS/SSL configuration guide/example to enable secure RAGFlow
deployments with proper certificate management.

#### Changes
- New HTTPS Setup Section: Step-by-step guide for SSL certificate
configuration
- Let's Encrypt Integration: Complete Certbot setup instructions
- Docker Configuration: Volume mapping examples for certificates

#### Key Features
- Prerequisites checklist
- Docker Compose configuration examples
- Support for both Let's Encrypt and existing certificates

#### Files Modified
- `README.md`
- `ragflow.https.conf` (new file)

docker/nginx/ragflow.https.conf

@@ -0,0 +1,41 @@
+server {
+    listen 80;
+    server_name your-ragflow-domain.com;
+    return 301 https://$host$request_uri;
+}
+
+
+
+server {
+    listen 443 ssl;
+    server_name your-ragflow-domain.com;
+
+    ssl_certificate /etc/nginx/ssl/fullchain.pem;
+    ssl_certificate_key /etc/nginx/ssl/privkey.pem;
+
+    root /ragflow/web/dist;
+
+    gzip on;
+    gzip_min_length 1k;
+    gzip_comp_level 9;
+    gzip_types text/plain application/javascript application/x-javascript text/css application/xml text/javascript application/x-httpd-php image/jpeg image/gif image/png;
+    gzip_vary on;
+    gzip_disable "MSIE [1-6]\.";
+
+    location ~ ^/(v1|api) {
+        proxy_pass http://ragflow:9380;
+        include proxy.conf;
+    }
+
+
+    location / {
+        index index.html;
+        try_files $uri $uri/ /index.html;
+    }
+
+    # Cache-Control: max-age~@~AExpires
+    location ~ ^/static/(css|js|media)/ {
+        expires 10y;
+        access_log off;
+    }
+}