GitHub Actions fail at "Deploy to GitHub Pages" for pull requests

[anthonyfok]

From https://github.com/OpenDRR/pathways-paper/runs/4613147866?check_suite_focus=true:

  /usr/bin/git push origin gh-pages
  remote: Permission to OpenDRR/pathways-paper.git denied to github-actions[bot].
  fatal: unable to access 'https://github.com/OpenDRR/pathways-paper.git/': The requested URL returned error: 403
  Error: Action failed with "The process '/usr/bin/git' failed with exit code 128"

It may be a good thing that git push origin gh-pages didn't work there because we do not want Pull Requests previews replacing the rendered document from the main branch. However, it would still be nice if we could preview the rendered document from a pull request.

TODO:

• Stop .github/workflows/asciidoctor.yml from failing: Do not run the "Deploy to GitHub Pages" step for pull requests.
• Find alternative way to still deploy pull requests to GitHub Pages but perhaps in a different directory