Split out HTML pages build into two steps

Author: ericmjl

.azure-pipelines/templates/nb-docs-win.yml

@@ -3,5 +3,9 @@ steps:
     activate bayesian-modelling-tutorial
     mkdir -p docs/notebooks
     jupyter nbconvert --config nbconvert_config.py --execute --template full
+  displayName: 'Convert notebooks to HTML'
+
+- script: |
+    activate bayesian-modelling-tutorial
     pandoc README.md -o docs/index.html -c static/pandoc.css -s
-  displayName: 'Build docs pages'
+  displayName: "Make index page."