Add 2.1 build script that reuses process from main branch

[kfranqueiro]

These changes are part 2 of 2 to enable preview builds against the WCAG-2.1 branch. This part applies to the WCAG-2.1 branch; See #4182 for part 1, which applies to main.

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.

I have temporarily stood up an example site using a version of this branch modified to run against the branch on my fork used for #4182:

https://kgf-wcag21-preview-test.netlify.app/

I have verified that the only diffs between the output of this script and directly running against the latest 2.1 publication from main are date stamps in page footers.

Here's what the deploy options look like for the netlify site: