CI: Specify that doc builds need extras

While we were already using the right dependencies, this didn't do any
of the extra steps (e.g. downloading maps).

.github/workflows/docs.yml

@@ -47,6 +47,7 @@ jobs:
       with:
         type: 'doc'
         python-version: ${{ matrix.python-version }}
+        need-extras: true
 
     - name: Build docs
       id: build-docs