### What problem does this PR solve?
Adds the documentation for three newly added API methods for content
management.
### Type of change
- [x] Documentation Update
### What problem does this PR solve?
Adds the api of listing documentation.
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### 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
