Setup Javadoc for fluss API discoverability

### Search before asking

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

### Description

We don't have the Javadoc yet, but doing so would be a major step forward for our developer ecosystem. It would provide a single, authoritative source of truth for our APIs, making it easier for developers to explore available classes, methods, and parameters without digging into the codebase. Beyond improving API discoverability, well-documented Javadocs speed up onboarding for new developers, reduce the learning curve, and lower the number of support queries.

### Willingness to contribute

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