Fix workflow: explicitly install ghp-import

Author: sun

.github/workflows/deploy-docs.yml

@@ -23,6 +23,8 @@ jobs:
         run: |
           git config --global user.name "github-actions[bot]"
           git config --global user.email "github-actions[bot]@users.noreply.github.com"
+          # Ensure ghp-import is available
+          pip install ghp-import
           mkdocs gh-pages --force
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}