### 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

### What problem does this PR solve?

This PR have completed both HTTP API and Python SDK for
'delete_dataset". In addition, there are tests for it.

### Type of change

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

### What problem does this PR solve?

This PR have completed both HTTP API and Python SDK for 'list_dataset".
In addition, there are tests for it.

### Type of change

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

### What problem does this PR solve?

This PR have finished 'create dataset' of both HTTP API and Python SDK.
HTTP API:
```
curl --request POST --url http://<HOST_ADDRESS>/api/v1/dataset   --header 'Content-Type: application/json' --header 'Authorization: <ACCESS_KEY>' --data-binary '{
  "name": "<DATASET_NAME>"
}'
```

Python SDK:
```
from ragflow.ragflow import RAGFLow
ragflow = RAGFLow('<ACCESS_KEY>', 'http://127.0.0.1:9380')
ragflow.create_dataset("dataset1")

```

TODO: 
- ACCESS_KEY is the login_token when user login RAGFlow, currently.
RAGFlow should have the function that user can add/delete access_key.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

---------

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