Content for mobile-content-responsiveness

As a visitor I want to experience features for that area.

AC: At least one example are given for:

Optimized mobile-first experience with responsive design and breakpoints in HTML5, CSS and JavaScript, and native interaction widgets—date picker, number pad, etc.—on smartphones and tablets.
Automatic, device-appropriate, and adaptive rendering delivers content optimized for users’ devices. Improve the user experience by delivering video, images, and documents in the best format, resolution, and bandwidth.
Achieve faster-loading pages and improved SEO ranking by delivering content using Google’s mobile-specific AMP (Accelerated Mobile Pages) format.
Responsive admin backend UI, allows full site and editorial management from any computer, tablet, or smartphone.
Built-in responsive preview simulates content as displayed on a wide variety of screen sizes and for specific devices.
A link is added at https://typo3.org/cms/features/frontend-experience#c9225 to https://typo3.org/cms/features/frontend-experience/mobile-content-responsiveness