[docs] Plan a fluss-quickstart-spark image for Spark Quickstart

### Search before asking

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

### Description

Parent issue: #2683

Now that Fluss has Spark connector support, we should plan a `apache/fluss-quickstart-spark` image for the Spark quickstart, similar in spirit to the proposed `fluss-quickstart-flink` image.

Scope:
- Define the goals and requirements for a Spark quickstart image
- Identify the dependencies that should be prebundled for a smooth Spark quickstart experience
- Decide how the image should align with the Spark quickstart documentation and release/versioning strategy
- Evaluate whether the Flink and Spark quickstart images can share conventions for naming, publishing, and maintenance

This issue is intentionally about planning and scoping first, so that Spark quickstart onboarding can follow the same low-friction model discussed in #2683.

### Willingness to contribute

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

Start by reading parent issue #2683 and the existing Spark quickstart documentation, then compare the proposed Flink quickstart image conventions. Done means documenting the image's goals, bundled dependencies, naming and publishing conventions, maintenance approach, and alignment with Spark quickstart versioning.

Written by the indexing model from the issue text.