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

### Search before asking

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

### 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!

Start by locating the existing Docker Compose setup and Fluss Gateway REST API documentation, then review the Flink SQL and Fluss Union Read examples. The quickstart is done when it starts Fluss, Fluss Gateway, Flink, Paimon, and RustFS; creates a lake-enabled table, ingests JSON events, verifies immediate reads and Paimon tiering, and documents limitations, write-result handling, and cleanup.

Written by the indexing model from the issue text.