Docs: Added a guide on configuring SSL certificates (#14696)

### What problem does this PR solve?

### Type of change

- [x] Documentation Update

docs/administrator/configurations/_category_.json

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+{
+  "label": "Configurations",
+  "position": 0,
+  "link": {
+    "type": "generated-index",
+    "description": "Guides for system configurations"
+  }
+}

docs/administrator/configurations/config_ssl_cert.md

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+---
+sidebar_position: 1
+slug: /config_ssl_cert
+sidebar_custom_props: {
+  categoryIcon: LucideCog
+}
+---
+# Configure SSL certificates
+
+Configure SSL certificates for a RAGFlow instance deployed via Docker.
+
+---
+
+This guide details how to configure SSL certificates for a RAGFlow instance deployed via Docker, using the container name `docker-ragflow-cpu-1` as an example.
+
+## 1. Prepare certificate files
+
+Ensure you have Nginx-formatted certificate files ready:
+
+- **Public Key**: Usually named `fullchain.pem` or `server.crt`.
+- **Private Key**: Usually named `privkey.pem` or `server.key`.
+
+If necessary, rename your files to match the standard:
+
+```bash
+# Rename bundle to fullchain.pem
+cp XXXXX_bundle.pem fullchain.pem
+# Rename private key to privkey.pem
+cp XXXXX.key privkey.pem
+```
+
+## 2. Confirm container status
+
+Verify that your container is running:
+
+```bash
+docker ps
+```
+
+## 3. Copy certificates to the container
+
+Transfer the files from your host machine to the container's temporary directory:
+
+```bash
+docker cp ./fullchain.pem docker-ragflow-cpu-1:/tmp/fullchain.pem
+docker cp ./privkey.pem docker-ragflow-cpu-1:/tmp/privkey.pem
+```
+
+## 4. Deploy certificates inside the container
+
+Enter the container's interactive terminal:
+
+```bash
+docker exec -it docker-ragflow-cpu-1 /bin/bash
+```
+
+Once inside, move the files and set appropriate permissions:
+
+```bash
+mkdir -p /etc/nginx/ssl
+mv /tmp/fullchain.pem /etc/nginx/ssl/
+mv /tmp/privkey.pem /etc/nginx/ssl/
+
+# Set permissions: 644 for public key, 600 for private key
+chmod 644 /etc/nginx/ssl/fullchain.pem
+chmod 600 /etc/nginx/ssl/privkey.pem
+```
+
+## 5. Switch Nginx to HTTPS configuration
+
+Replace the default HTTP configuration with the HTTPS template:
+
+1. Navigate to the configuration directory: `cd /etc/nginx/conf.d/`.
+2. Back up the original configuration: `mv ragflow.conf ragflow.conf.bak`.
+3. Enable the HTTPS template: `cp /etc/nginx/ragflow.https.conf ./ragflow.conf`.
+
+## 6. Edit the HTTPS template
+
+1. Open the configuration file: `vi ragflow.conf`.
+2. Ensure `ssl_certificate` and `ssl_certificate_key` paths point to your files in `/etc/nginx/ssl/`.
+3. Verify the Nginx syntax: `nginx -t`.
+
+## 7. Apply the configuration
+
+Reload Nginx to apply changes:
+
+```bash
+nginx -s reload
+```
+
+If the changes do not take effect, exit the container and restart it:
+
+```bash
+exit
+docker restart docker-ragflow-cpu-1
+```
+
+## Configuration persistence
+
+:::tip IMPORTANT
+Changes made via `docker cp` and `docker exec` are lost if the container is removed or stopped via `docker-compose down`.
+**Recommendation**: After a successful test, store the certificates on the host machine and use `volumes` in your `docker-compose.yaml` to mount the certificates and `ragflow.conf` permanently.
+:::