publish docs: Install missing sassc

Author: jelmer

.github/workflows/build-upload-docs.yml

@@ -45,6 +45,9 @@ jobs:
       - name: Install sphinx python package from pip
         run: pip install -U -r sphinx-requirements.txt
 
+      - name: Install sassc for CSS compilation
+        run: sudo apt-get update && sudo apt-get install -y sassc
+
       - name: Build docs
         run: make site