Rebuild HTML on Bootstrap 5

The new HTML does not exactly match the old HTML, but it is close in
style. The largest differences are:

- The TOC is available via a fixed dropdown at the top of the screen. I
 think this is much more usable.

- Blue is used as a accent color, instead of green. I prefer the blue,
but it doesn't match the illustrations. This may be revisited in the
future.

Other changes:

- CSS built from SCSS (Sass) instead of Less
- TOC depth limited to 2, which I think is much more usable

build.sbt

@@ -182,7 +182,7 @@ pdfSetup := {
 htmlSetup := {
   "mkdir -p dist src/temp".!
   "npm install".!
-  "npx lessc --include-path=node_modules --strict-imports src/less/html.less src/temp/html.css".!
+  "npx sass --load-path=node_modules src/scss/html.scss src/temp/html.css".!
   "npx browserify src/js/html.js --outfile src/temp/html.js".!
 }