Go for a more generic js bundling architecture

* Don't tie the output name to webpack itself
* Don't call js bundling webpack in python code
* Make the doc more generic about js bundling

pedagogy/templates/pedagogy/guide.jinja

@@ -10,7 +10,7 @@
 {% endblock %}
 
 {% block additional_js %}
-  <script src="{{ static('webpack/pedagogy/guide-index.js') }}" defer></script>
+  <script src="{{ static('bundled/pedagogy/guide-index.js') }}" defer></script>
 {% endblock %}
 
 {% block head %}