Setup Javadoc for fluss API discoverability

[MehulBatra]

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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

• I'm willing to submit a PR!