Implement a GitLab UI starter recipe

In order to help users create projects that use @gitlab/ui, we should implement a starter recipe.

This would support Implement a Claude Project for creating prototy... (#3209), and also regular human experimentation.

One or both of these approaches would work:

• A full-featured starter template (e.g., see https://github.com/vitejs/awesome-vite?tab=readme-ov-file#templates).
  • With this everything would work, including full Tailwind support.
• A minimal HTML starter file template.
  • Some things wouldn't work, like arbitrary Tailwind classes.

I'm inclined to go with the full-featured starter template, since it's more realistic, useful and complete.

This should also be documented in https://design.gitlab.com somewhere.