[Data][Docs] Fix inconsistent heading levels in "How to write tests" guide

[bveeramani]

Description

The Ray Data "How to write tests" guide has inconsistent heading levels that break the document's logical structure. Several sections that should be nested under "Ray-specific practices" are incorrectly using H2 (##) instead of H3 (###).

Background

The guide is located at doc/source/data/contributing/how-to-write-tests.md. It's organized into two main sections:

• "General good practices" (H2) with subsections (H3)
• "Ray-specific practices" (H2) with subsections (H3)

However, the last three sections of the document use H2 headings:

• ## Avoid testing against repr outputs to validate specific data
• ## Avoid assumptions about the number or size of blocks
• ## Avoid testing that the DAG looks a particular way

These sections discuss Ray Data-specific concepts (ds.stats(), ds._plan, DAG structure) and logically belong under "Ray-specific practices" as H3 subsections.

Motivation

Consistent heading structure:

1. Makes the document easier to navigate
2. Renders correctly in documentation table of contents
3. Clearly communicates the relationship between sections

Implementation Boundaries & Constraints

• Only change the heading levels—don't rewrite content
• Verify the rendered documentation looks correct after the change

[yuhuan130]

@bveeramani I would love to take this problem. Thank you.