[docs] Add HTTP Gateway lakehouse quickstart

[pranavshuklaa]

Purpose

Linked issue: close #4221

Add an end-to-end quickstart showing how to build a real-time lakehouse using the Fluss HTTP Gateway, Fluss, Flink, Paimon, and RustFS.

Brief change log

• Add a Docker Compose setup including the Fluss Gateway.
• Document building the Gateway image locally.
• Show how to create a datalake-enabled Fluss table through the Gateway REST API.
• Show how to ingest records through the Gateway HTTP API.
• Show how to start the Lakehouse Tiering Service and tier data to Paimon.
• Demonstrate querying tiered data through $lake and $lake$snapshots.
• Demonstrate Union Read for the current unified Fluss + Paimon view.
• Document Gateway preview limitations, write-result handling, readiness, and cleanup.

Tests

• Verified the Gateway Docker image builds successfully in GitHub Actions.
• Validated the Docker Compose configuration and the documented Gateway REST API workflow during development.
• Verified the Fluss Docker build and quickstart environment setup through GitHub Actions.

API and Format

No API or storage format changes.

Documentation

Adds a new HTTP Gateway quickstart for a real-time lakehouse and documents the existing Gateway REST API, Fluss datalake integration, Flink Union Read, and Paimon tiering workflow.

[pranavshuklaa]

Could someone please review this PR when you get a chance? It adds the HTTP Gateway lakehouse quickstart for #4221. Thanks!!