### What problem does this PR solve?

Added get_dataset and update_dataset API.
Fixed delete_dataset.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

### 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
- [x] Documentation Update

### 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

- [x] Documentation Update

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

### What problem does this PR solve?

This PR added ragflow_api.md and more tests for API.

### Type of change

- [x] Documentation Update
- [x] Other (please describe): tests