[UPDATED] Slide Shows

[Rob Beezer]

A #slideshow is now in the schema. And maybe more interestingly, the brief
description in the Guide has been expanded some. See

Section 4.43: Slides
https://pretextbook.org/doc/guide/html/topic-slides.html#topic-slides

Remember, there are two conversions: one to a Beamer LaTeX/PDF, another to an
HTML Reveal.js slideshow. And there is a sample slideshow in examples/ that you
can experiment with. It is built into both outputs and included at the website.

Rob

https://github.com/PreTeXtBook/pretext/pull/3135