Add GitHub action to publish 2.1 ED updates

[kfranqueiro]

This adds an action to the WCAG-2.1 branch to update files on the gh-pages branch related to 2.1 guidelines, which reside directly under guidelines/ (not under a subfolder). Prior to this, the editor's draft for WCAG 2.1 has not been updated since shortly after its first publication.

• Test commit on fork used to trigger action
• Successful action run on fork
• gh-pages commit produced by the action
• Diff between ED on w3c.github.io vs. latest update