Write handout building script

[pintergreg]

• [x] preprocess slides with pandoc
• build site with mdbook
• write custom mdbook CSS to hide unwanted elements
• write filter to exclude unwanted elements
  • [x] quiz
  • [x] duplicated header
• [x] copy figures
• [x] GH Actions script

[pintergreg]

Custom CSS won't work to hide some elements, because commonmark transcription removes the classes.

[pintergreg]

Instead of trying to exclude a slide, use a fenced div for the quiz content to exclude.

[pintergreg]

The mdbook execution should not be included in the script, the CI script can handle that.

[pintergreg]

No mdbook in Ubuntu repo, solved via this: https://lindevs.com/install-mdbook-on-ubuntu