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)
