added build befor gh-deploy

Author: haeussma

.github/workflows/gen.yml

@@ -59,4 +59,5 @@ jobs:
           path: .cache
           key: ${{ github.ref }}
       - run: pip install mkdocs-material jupyter nbconvert mkdocs-jupyter mkdocs-plugin-inline-svg
+      - run: mkdocs build
       - run: mkdocs gh-deploy --force