Create Summary box pattern

Added pattern.

Also included a css library for overriding certain things on the /patterns page.

For example we are using in-page navigation which picks up all headers. So the preview for summary has an h3 that makes no sense in the navigation. There's an open issue on uswds to be able to exclude headers. But hiding for now.

Automatically closed - issue fixed for 2 weeks with no activity.

smustgrave → credited pdureau → .

Review in the meta.