Add SAM.gov resources to the implementations documentation

[afeijoo]

Consider adding SAM.gov resources for their design system theme, Angular component library, and prototypes to the USWDS implementations page.

• Sam-Styles - https://github.com/GSA/sam-styles - HTML/CSS design library built on USWDS 2.0 and Fractal.
• Sam-Design-System - https://github.com/GSA/sam-design-system - Common Angular components integrated with sam-styles css framework.
• Sam-Prototypes - https://github.com/GSA/sam-prototypes - A test application that incorporates sam-styles and sam-design-system components with real-world examples.