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.
Add one presentation that provides a very simple demo, using
impress.js to create a very traditional "slide show". Possibly
it's an easier way to learn impress.js (it's commented, just like
the official demo.) It uses the relative positioning plugin and
uses "speaker notes", which aren't shown in the presentation,
but are picked up and shown in the speaker console (press 'P').
Also uses autoplay, forms... and includes short demo of all the extra
addons from extras/. (Highlight.js, Markdown.js, Mathjax.js, Mermaid.js)
