Install sphinxext-rediraffe in the install process.

.github/workflows/ci.yaml

@@ -21,6 +21,7 @@ jobs:
           python -m pip install -e .[dev]
           python -m pip install -e .[test]
           python -m pip install ltd-conveyor
+          python -m pip install sphinxext-rediraffe
       - name: Build documentation
         run: |
           cd doc