Pin docs workflow to v20

Author: jeromekelleher

.github/workflows/docs.yml

@@ -15,7 +15,7 @@ permissions:
 
 jobs:
   Docs:
-    uses: tskit-dev/.github/.github/workflows/docs.yml@v19
+    uses: tskit-dev/.github/.github/workflows/docs.yml@v20
     with:
       pyproject-directory: python
       additional-apt-packages: doxygen