Create guide and evals for the accessibility use case

[chrome-devrel-infra]

Actionable coding guidelines for building accessible web applications, covering semantic HTML, focus management, forms, media, and testing. Use this skill when auditing or implementing accessibility features, keyboard navigation, or ARIA.

Affected web-feature IDs:

Use case subdir: guides/accessibility/accessibility

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