Add format script

Runs HTML Tidy.

(The expected version is tidy-html5 from https://www.html-tidy.org/)

scripts/format.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+set -eu
+cd "$(dirname "$0")"
+cd ..
+echo "Entering directory '$PWD'"
+set -x
+tidy \
+    --indent yes \
+    --quiet yes \
+    --show-filename yes \
+    --tidy-mark no \
+    --write-back yes \
+    www/*.html