Use GHA bot user

from:actions/checkout#13 (comment)

Author: llrs

Diff for: .github/workflows/pkgdown.yaml

@@ -51,3 +51,5 @@ jobs:
           clean: false
           branch: gh-pages
           folder: docs
+          git-config-name: github-actions[bot]
+          git-config-email: 41898282+github-actions[bot]@users.noreply.github.com