C++ Client documentations should have instruction to download released package

### Search before asking

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

### Description

C++ doc currentrly only have instruction to build from source, we should also update it so that there's instruction to pull released C++ binary (using standard C++ convention)

website/docs/user-guide/cpp/installation.md

### Willingness to contribute

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