break out HTML page building into two steps

Author: ericmjl

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

@@ -2,6 +2,10 @@ steps:
 - script: |
     source activate bayesian-modelling-tutorial
     mkdir -p docs/notebooks
-    jupyter nbconvert --config nbconvert_config.py --execute --template full &&
-    pandoc README.md -o docs/index.html -c static/pandoc.css -s &&
-  displayName: 'Build docs pages'
+    jupyter nbconvert --config nbconvert_config.py --execute --template full
+  displayName: 'Convert notebooks to HTML'
+
+- script: |
+    source activate bayesian-modelling-tutorial
+    pandoc README.md -o docs/index.html -c static/pandoc.css -s
+  displayName: "Make index page."