Illustration: Needs content written regarding usage

[jenn-franklin]

Which page is this about? Illustration: https://cfpb.github.io/design-system/foundation/illustration

Describe your issue

• Write content regarding when we should we use illustrations in our work. When doesn't a product need an illustration? When should we use photography instead of illustration? Etc.
• Any info we can add to better help folks understand how to design within our illustration style and incorporate them into our publications?
• Link to components that require illustration so you can access those specs easily.

[jenn-franklin]

This issue was created when we migrated the Design Manual to our current Design System site. During that time, we reviewed all pages to identify potential improvements. These are nice ideas, but they have not been reported by our team's designers or developers as being critical to implement. Because of this, we've decided to close this issue in favor of prioritizing issues that have been reported by our designers and developers as being blockers to their work. Thanks!