Add 2.1 build script that reuses process from main branch (#4183)

This adds a shell script which downloads a tarball of the latest `main`
branch, extracts it to a subfolder, then runs the Eleventy build within
it, instructing it to build against 2.1 but using the local `guidelines`
folder from the active branch rather than the latest published version.

build-21.sh

@@ -0,0 +1,10 @@
+#! /bin/sh
+
+# Download main branch
+curl -Lo wcag.tar.gz https://github.com/w3c/wcag/archive/refs/heads/main.tar.gz
+tar xf wcag.tar.gz
+cd wcag-main
+npm i
+
+# Run build using local 2.1 guidelines from active branch
+WCAG_VERSION=21 WCAG_FORCE_LOCAL_GUIDELINES=../guidelines/index.html npm run build