Update workflow.

- Install pandoc (required).
- Don't upgrade PIP, takes time, and might not be required to do so
  all the time.

.github/workflows/docs_check.yaml

@@ -23,7 +23,8 @@ jobs:
 
     - name: Install dependencies
       run: |
-          python -m pip install --upgrade pip
+          sudo apt-get update
+          sudo apt-get install -y pandoc
           python -m pip install -e .[development]
 
     - name: Check for Sphinx warnings