replace workflow png by svg

content/index.txt

@@ -20,7 +20,7 @@ Python's simple and very expressive syntax) to define the system and calculate
 its quantum properties (conductance, density of states, etc). This workflow is
 summarized as follows:
 
-.. image:: kwant_workflow.png
+.. image:: kwant_workflow.svg
     :width: 90%
 
 Kwant was designed to be easy to use: for example the program that generates