Add documentation and examples for scaling to larger screens.

Document all attributes to root element (#impress) in DOCUMENTATION.md.

For the main demo (/index.html) add width, height, max-scale, min-scale
attributes to the main demo. These just use the defaults, except
data-max-scale="3" which allows to scale to larger screens (HD, 4K).

For examples/classic-slides, add attributes to root element and set it
to target Full HD 1920 x 1080 resolution. Adjust CSS to match.

Add notification everywhere that defaults will change in 2021.

css/impress-demo.css

@@ -205,6 +205,12 @@ a:focus {
     ;)
 
     They cover everything what you see on first three steps of the demo.
+
+    All impress.js steps are wrapped inside a div element of 0 size! This means that relative
+    values for width and height (example: `width: 100%`) will not work. You need to use pixel
+    values. The pixel values used here correspond to the data-width and data-height given to the
+    #impress root element. When the presentation is viewed on a larger or smaller screen, impress.js
+    will automatically scale the steps to fit the screen.
 */
 .slide {
     display: block;