Update deploy workflow to use Eleventy build process

[kfranqueiro]

This replaces the existing files under .github (both workflows and scripts) with a new workflow using the Eleventy build process.

• Current behavior RE conformance-challenges, guidelines, and requirements directories is retained
  • Also ensures necessary CSS is updated under guidelines
• techniques, understanding, and working-examples are now generated by the Eleventy build process

This has positive side effects over what's currently seen on https://w3c.github.io/wcag/techniques/ and https://w3c.github.io/wcag/understanding/:

• Fixes broken styles
• Updates any auto-generated cross-links to guidelines to point to 2.2 editor's draft instead of 2.1
• All of the fixes mentioned in Implement Eleventy-based build system #3917

Note that this intentionally does not clean out old files under techniques, understanding, and working-examples (as mentioned in #3952 and seen rolled back in the 2nd commit in this PR). There are future plans to bring back obsolete techniques pages with some indication of their obsolescence; performing this cleanup now would lead to a lot of files being deleted now, only to be recreated later.

Previews:

• https://kfranqueiro.github.io/wcag/techniques/
• https://kfranqueiro.github.io/wcag/understanding/

[w3cbot]

kfranqueiro marked as non substantive for IPR from ash-nazg.

[iadawn]

Looks good to me