Try pandoc instead of marp

[jonathanpallant]

pandoc does a better job of parsing markdown than marp does. In particular, it has the rule:

A header at the slide level always starts a new slide.

This means we don't need to add horizontal breaks (---) manually to the slides (which would spoil the mdbook output).

pandoc can output:

• slideous (Slideous HTML and JavaScript slide show)
• slidy (Slidy HTML and JavaScript slide show)
• dzslides (DZSlides HTML5 + JavaScript slide show)
• revealjs (reveal.js HTML5 + JavaScript slide show)
• s5 (S5 HTML and JavaScript slide show)

[jonathanpallant]

Paging @Mirabellensaft and @miguelraz