[Docs] Add documentation for C++ Client

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.


### Description

We should have documentation for C++ client similar to what we have for Java client

https://fluss.apache.org/docs/apis/java-client/

### Willingness to contribute

- [ ] I'm willing to submit a PR!