Add page explaining how to send template using API

Would like to test something like this and see how well it works.

Intention of having this page is so:
- template IDs are discoverable (https://www.pivotaltracker.com/story/show/115404593)
- it’s obvious there’s an ‘automated’ way to send messages, as well as the CSV
  way (we’ve seen people oblivious to this in research)

app/templates/views/documentation.html

@@ -9,7 +9,7 @@
 {% block maincolumn_content %}
 
 <h1 class="heading-large">
-    Developer documentation
+    Set up API integration
 </h1>
 
 <div class="grid-row">