Fix spacing of first page headers

This commit customises the margins of the first header on each page so that it
lines up with the navigation.

app/templates/views/documentation.html

@@ -8,13 +8,13 @@ GOV.UK Notify | API keys and documentation
 
 {% block maincolumn_content %}
 
+<h1 class="heading-large">
+    Developer documentation
+</h1>
+
 <div class="grid-row">
     <div class="column-two-thirds">
 
-        <h1 class="heading-large">
-            Developer documentation
-        </h1>
-
         <h2 class="heading-medium">
             How to integrate GOV.UK Notify into your service
         </h2>