Fix deploy.yml: preserve preview directories on main deploy

Without keep_files: true, peaceiris/actions-gh-pages replaces the
entire gh-pages branch on every main push, wiping any preview/pr-N/
subdirectories deployed by the PR preview workflow.

Author: ajithsureshtii

.github/workflows/deploy.yml

@@ -26,3 +26,4 @@ jobs:
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./_site
+          keep_files: true