Make a reusable ‘content’ template

This is for static content pages and makes sure they always have the
right column widths and sub navigation.

app/templates/views/documentation.html

@@ -1,16 +1,13 @@
-{% extends "withoutnav_template.html" %}
+{% extends "content_template.html" %}
 
 {% block per_page_title %}
   Documentation
 {% endblock %}
 
-{% block maincolumn_content %}
-
-<div class="grid-row">
-  <div class="column-two-thirds">
+{% block content_column_content %}
 
   <h1 class="heading-large">Documentation</h1>
-    <p>Use the GOV.UK Notify API to send messages automatically.</p> 
+    <p>Use the GOV.UK Notify API to send messages automatically.</p>
   <h2 class="heading-medium">Clients</h2>
     <p>Choose a client to integrate our API with your web application or back-office system. Links to documentation open in a new tab.</p>
   <ul class="list list-bullet">
@@ -25,7 +22,5 @@
       <li><a href="https://docs.notifications.service.gov.uk/{{ key }}.html" target="_blank" rel="noopener">{{ label }}</a></li>
     {% endfor %}
   </ul>
-  </div>
-</div>
 
 {% endblock %}