Create guide and evals for the css use case

[chrome-devrel-infra]

Action-oriented guidelines for modern CSS architecture, layouts, and performance. Use this guide when authoring styles, managing design systems, or optimizing web rendering.

Affected web-feature IDs:

Use case subdir: guides/css/css

Required files:

• Use case metadata (guide.md frontmatter)
• demo.html
• Use case guidance (guide.md)
• expectations.md
• tasks/task.md
• negative-demo.html
• grader.ts