[docs] Add an HTTP Gateway quickstart for a real-time lakehouse

[beryllw]

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Description

Add an end-to-end quickstart that demonstrates how an application can send JSON events through Fluss Gateway and make the same data available for both real-time queries and lakehouse analytics.

The quickstart should:

• start Fluss, Fluss Gateway, Flink, Paimon, and RustFS with Docker Compose;
• create a lake-enabled Fluss table through the Gateway REST API;
• ingest a batch of JSON events through HTTP;
• query fresh rows immediately through Flink SQL and Fluss Union Read;
• verify that the rows are continuously tiered to Paimon;
• document preview limitations, write-result handling, and cleanup steps.

Willingness to contribute

• I'm willing to submit a PR!

[pranavshuklaa]

Hi, I’m planning to take up this quickstart doc and work on the Docker Compose setup and documentation. I’ll start working on it and raise a PR once it’s ready and tested too.

[pranavshuklaa]

I've opened a PR #4231 for this issue, adding the HTTP Gateway lakehouse quickstart and covering the end-to-end Gateway -> Fluss -> Flink -> Paimon flow. Would appreciate a review when someone gets a chance. Thanks!

[Gezi-lzq]

Hi @beryllw, I opened PR #3851 a while ago to introduce a contributor-focused local DevKit, and recently added a Fluss Gateway scenario that is related to the quickstart you described in this issue. If you’re interested, I’d really appreciate your review and feedback. (I also happened to uncover the issue fixed in PR #4205 while integrating and testing Gateway.)