CCM-11937 Add maincontent ID to 404 page

Author: nicki-nhs

docs/pages/404.html

@@ -18,9 +18,9 @@
   }
 </style>
 
-<div class="container">
+<div class="container" id="maincontent">
   <h1>404</h1>
 
   <p><strong>Page not found :(</strong></p>
   <p>The requested page could not be found.</p>
-</div>
+</div>