fix(ci): skip docs preview deploy for fork PRs (#679)

Fork PRs receive a read-only GITHUB_TOKEN that cannot push to gh-pages,
causing the deploy step to fail with a 403. Skip the deploy for fork PRs
while still running the docs build to validate changes compile cleanly.

Author: johntmyers

.github/workflows/docs-preview-pr.yml

@@ -49,6 +49,7 @@ jobs:
                   find _build -name .buildinfo -exec rm {} \;
 
             - name: Deploy preview
+              if: github.event.pull_request.head.repo.full_name == github.repository
               uses: rossjrw/pr-preview-action@v1
               with:
                   source-dir: ./_build/docs/