Add grayscale alert-tip to css framework

What does this MR do?

This MR adds the class alert-tip to our css framework. Tip alerts are referenced in the GitLab design system but do not exist in our code yet.

Why do we need this?

This is referenced in the UX designs for gitlab-ee#5426. Here's how alert-tip is used in the feature branch of gitlab-ee!7727.

What are the relevant issue numbers?

Part of gitlab-ee#5426

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer