Switch deploy from automatic github pages to deploy that is done with custom github action

[dmundra]

The automatic github pages deploy intermittently fails e.g. https://github.com/CivicActions/accessibility/runs/6712655812?check_suite_focus=true. Tracking down the error I found that this is likely caused by the Jekyll version (3.9.2) and sass race conditions (see cloud-gov/pages-uswds-jekyll#223). Looks like the default github pages deploy is locked into that version (see github/pages-gem#651 (comment)). So recommendation is to do own GitHub actions deploy where we build the site with Jekyll version 4 (like we do the QA site) and deploy it to a branch 'gh-pages' and then use that branch to render the site.