install requirements in documentation workflow

Author: simoneliuzzo

.github/workflows/documentation.yml

@@ -47,6 +47,9 @@ jobs:
       - name: Install package
         run: python -m pip install '.[docs]'
 
+      - name: Install dependencies
+        run: pip install -r ./docs/requirements.txt
+        
       - name: Build documentation
         run: python -m sphinx -b html docs ./docs/_build -d ./docs/_build