Add docs dependencies to tests CI

When we test the documentation, we need its dependencies.

.github/workflows/ci.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Update pip and install dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install ".[test]"
+          python -m pip install ".[docs,test]"
 
       - name: Run tests
         run: |