Provide HiDPI versions of images in documentation

We should have @2x versions of all images in our documentation so that they look nice and sharp on high-DPI devices like 4K monitors, MacBook Pros, and Microsoft's line of Surface devices.

HTML5 even has the srcset attribute for dealing with multi-resolution images on the web these days:

• https://webkit.org/demos/srcset/
• https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images