Add further packages that need to be installed for mkdocsx

.github/workflows/build_docs.yml

@@ -24,6 +24,5 @@ jobs:
           path: .cache
           restore-keys: |
             mkdocs-material-
-      - run: pip install mkdocs-material
-      - run: cd documentation
+      - run: pip install mkdocs-material pymdown-extensions mkdocstrings-python
       - run: mkdocs gh-deploy --force