Fix documentation workflow

.github/workflows/documentation.yml

@@ -20,6 +20,11 @@ jobs:
       run: |
         python -m pip install --upgrade pipenv
         pipenv install --dev
-    - name: Build and deploy documentation
-      run: |
-        pipenv run mkdocs gh-deploy
+    - name: Build
+      run: mkdocs build
+    - name: Deploy
+      uses: peaceiris/actions-gh-pages@v2.5.0
+      env:
+        ACTIONS_DEPLOY_KEY: ${{ secrets.ACTIONS_DEPLOY_KEY }}
+        PUBLISH_BRANCH: gh-pages
+        PUBLISH_DIR: ./site