ci: add missing pip dependency

.github/workflows/ci.yml

@@ -12,5 +12,5 @@ jobs:
       - uses: actions/setup-python@v2
         with:
           python-version: 3.x
-      - run: pip install mkdocs-material mkdocs-git-revision-date-localized-plugin mkdocs-awesome-pages-plugin
+      - run: pip install mkdocs-material mkdocs-git-revision-date-localized-plugin mkdocs-awesome-pages-plugin mkdocs-callouts
       - run: mkdocs gh-deploy --force