Create guide and evals for the contrast-color use case #961

Select a readable text color with sufficient contrast on a dynamic background color.

Affected web-feature IDs: contrast-color

Use case subdir: guides/visual-design/contrast-color

Related features: #687

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