### What problem does this PR solve?
As RAGFlow has an integration with Langfuse, this docs page shows how to
configure Langfuse tracing.
### Type of change
- [ ] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):
