fixup: install missing dependency

.gitlab-ci.yml

@@ -20,7 +20,7 @@ build and upload the contents:
     - chmod 700 ~/.ssh
     - ssh-keyscan tnw-tn1.tudelft.net >> ~/.ssh/known_hosts
     - chmod 644 ~/.ssh/known_hosts
-    - pip install -U mkdocs mkdocs-material python-markdown-math
+    - pip install -U mkdocs mkdocs-material python-markdown-math notedown
 
   script:
     # Compile lectures