### 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)
