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.
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