[Doc][ftp] ftp connector use demo and doc

### Search before asking

- [X] I had searched in the [issues](https://github.com/DTStack/chunjun/issues) and found no similar document issue.

### Description

当前ftp 插件的文档还没补，需要添加ftp 文档和使用案例

### Documentation Links

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)