About page: Imagery and beautification.

Still to come: responsive images, optimized videos, and CDN URLs.

props melchoyce, helen, ramiabraham.
see #38616.

Built from https://develop.svn.wordpress.org/trunk@39512


git-svn-id: https://core.svn.wordpress.org/trunk@39452 1a063a9b-81f0-0310-95a4-ce76da25c4cd