docs: Improve Spark documentation navigation and structure to align with other Kubeflow SDK components

What would you like to be added?

The goal of this issue is to improve the Spark documentation by restructuring it into multiple sections, similar to the Trainer and Optimizer documentation.

Potential improvements include:

• Add a Example section.
• Split the documentation into dedicated guides instead of a single page.
• Improve the current documentation

This issue can be implemented through multiple documentation PRs.

Why is this needed?

The Spark documentation currently consists of a single navigation entry, making it difficult to discover features and navigate as the SDK grows.

Trainer and Optimizer already provide a structured documentation experience with dedicated sections and guides. Aligning the Spark documentation with that structure will improve discoverability, onboarding, and overall consistency across the Kubeflow SDK documentation.