Point setup-python workflow step at mkdocs-reqs.txt

.github/workflows/publish-docs.yml

@@ -55,6 +55,7 @@ jobs:
         with:
           python-version: '3.13'
           cache: pip
+          cache-dependency-path: mkdocs-reqs.txt
 
       - name: Ensure mkdocs is available
         run: make ensure-mkdocs