Build slides with gh actions

Key points from trial and error:
- It seems that using a docker image is the recommended approach for pandoc.
- As such, I can't use my Makefile to build the slides, so need to duplicate
  the arguments in the action.
- The path to the css file needs to be relative to the output directory,
  rather than the directory you run the command from,
  so here it's `-css=mywhite.css`.
- The github-pages-deploy-action needs to deploy the root directory,
  because most of the site is built using jekyll, and if you just deploy the
  the slideshow directory, it deletes the rest of the site!

.github/workflows/website.yml

@@ -125,3 +125,13 @@ jobs:
 
       - run: make lesson-check-all
         if: always()
+
+      - name: Build slides
+        uses: docker://pandoc/core:latest
+        with:
+          args: --to=revealjs --standalone -o slideshow/index.html --css=mywhite.css  slideshow/slides.md
+
+      - name: Deploy slides
+        uses: JamesIves/[email protected]
+        with:
+          folder: .