Modified the chart to retain persistent volumes by default when the
chart is uninstalled, following established best practices in the Helm
community (e.g., Bitnami charts)

### What problem does this PR solve?

Previously, deleting the helm chart would automatically remove all
persistent data, which poses a risk of accidental data loss.

### Rationale

This change aligns with industry standards to safeguard data by
requiring explicit action to remove persistence, rather than making
deletion the default behavior.

### Impact: 

Users who intentionally want to remove persistent data will need to do
so manually or by setting appropriate flags during chart uninstallation.

### Type of change

- [ ] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

### What problem does this PR solve?

Add's a Helm chart for deploying RAGFlow on Kubernetes. 

Closes #864. 

### Type of change

- [X] New Feature (non-breaking change which adds functionality)