Use dependency group in docs build too!

Author: samcunliffe

.github/workflows/docs.yml

@@ -31,7 +31,7 @@ jobs:
       - name: Install Dependencies
         run: |
           python -m pip install --upgrade pip
-          python -m pip install ".[docs]"
+          python -m pip install "." --group docs
           sudo apt install graphviz --yes
 
       - name: Build Docs