[fix] doc generation on github

Author: fracape

.github/workflows/static-gh-pages.yml

@@ -10,6 +10,8 @@ jobs:
     name: publish documentation
     steps:
       - uses: actions/checkout@v2
+        with:
+          submodules: recursive
       - name: Setup Python
         uses: actions/setup-python@v2
         with: