Rename Markdown files to have an .md extension. (#209)

Author: daveverwer

getting-started/_go-further.html renamed to getting-started/_go-further.md

@@ -4,5 +4,5 @@ title: Getting Started
 ---
 
 {% include getting-started/_installing.md %}
-{% include_relative _use-cases.html %}
-{% include_relative _go-further.html %}
+{% include_relative _use-cases.md %}
+{% include_relative _go-further.md %}