[docs] Move the manual JAR + Docker Compose Flink setup into Engine Flink getting started

### Search before asking

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

### Description

Parent issue: #2683

The current manual workflow for the Flink quickstart downloads multiple JARs and mounts them into a generic Flink Docker image. That path is still useful for advanced users who want more control over versions and dependencies, but it is no longer a good default for the main quickstart.

We should move that workflow into an advanced "Engine Flink -> Getting Started" guide and position it as a customizable setup path.

Scope:
- Move the existing manual JAR download + Docker Compose instructions out of the main Flink quickstart
- Add the workflow to the Engine Flink getting started documentation, or a closely related advanced setup page
- Keep the guide focused on users who need manual dependency management or custom Flink image composition
- Cross-link this advanced path from the main quickstart for users who need more flexibility

This split keeps the quickstart simple while preserving the more customizable setup flow requested in #2683.

### Willingness to contribute

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

Locate the current Flink quickstart and the Engine Flink getting started documentation, then identify the manual JAR download and Docker Compose instructions in the quickstart. Move that workflow into an advanced setup section, explain its customizable dependency and image use, and add a cross-link from the main quickstart. Done means both paths are clearly separated and the documentation links work.

Written by the indexing model from the issue text.