docs: add CSS data guidelines

[BHUVANSH855]

Summary

• Add CSS-specific BCD data guidelines.
• Document how to model CSS at-rules, properties, selectors, and types.
• Clarify when CSS types, values, functions, and syntax should be represented as separate features or subfeatures.
• Document that specification grammar alone is not sufficient reason to create a BCD feature.
• Add examples based on existing CSS compatibility data and recent BCD discussions.

Test results and supporting details

• npm run format — passed after formatting the new guideline.
• npm run lint — passed; all BCD data passed linting.
• eslint — 0 errors; existing warnings only.
• tsc --noEmit — passed.
• Prettier — passed.
• git diff --cached --check — passed.

The guidelines were informed by existing CSS BCD data and the discussions in #28838, #29034, and #6349.

Related issues

Fixes #28913

Related:

• css.types.timeline-range-name - Missing compatibility data #28838
• Chrome 146 scroll-triggered animations #29034
• Normalize references to CSS data types in feature object #6349

[github-actions]

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