Remove documentation pages

These are out of date now and potentially misleading.

Adds a new error page for HTTP `410 GONE`. Looks like the 404 page, but
returns the appropriate error code to be a good web citizen.

app/templates/error/410.html

@@ -0,0 +1,14 @@
+{% extends "withoutnav_template.html" %}
+{% block page_title %}Page not found{% endblock %}
+{% block maincolumn_content %}
+    <div class="grid-row">
+      <div class="column-two-thirds">
+        <h1 class="heading-large">
+          Page could not be found
+        </h1>
+        <p>
+          Check you’ve entered the correct web address.
+        </p>
+      </div>
+    </div>
+{% endblock %}