install mkdocs-material on workflow

.github/workflows/docs.yml

@@ -25,7 +25,8 @@ jobs:
         with:
           fetch-depth: 0
       - uses: actions/setup-python@v2
-      - run: pip install --upgrade pip && pip install mkdocs mkdocs-gen-files
+      - run: |
+          pip install --upgrade pip && pip install mkdocs mkdocs-material mkdocs-gen-files
       - run: git config user.name 'github-actions[bot]' && git config user.email 'github-actions[bot]@users.noreply.github.com'
       - name: Publish docs
         run: mkdocs gh-deploy