Add a separate page for linking to documentation

It’s going to get too cluttered to have these links on the API index
page.

app/templates/views/api/documentation.html

@@ -0,0 +1,35 @@
+{% extends "withnav_template.html" %}
+{% from "components/table.html" import list_table, field, hidden_field_heading %}
+{% from "components/api-key.html" import api_key %}
+{% from "components/page-footer.html" import page_footer %}
+
+{% block page_title %}
+  API integration – GOV.UK Notify
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <h1 class="heading-large">
+    Documentation
+  </h1>
+
+  <ul class="list list-bullet">
+    {% for name, url in [
+      ('Java client', 'https://github.com/alphagov/notifications-java-client'),
+      ('Node JS client', 'https://github.com/alphagov/notifications-node-client'),
+      ('PHP client', 'https://github.com/alphagov/notifications-php-client'),
+      ('Python client', 'https://github.com/alphagov/notifications-python-client'),
+      ('Ruby client', 'https://github.com/alphagov/notifications-ruby-client')
+    ] %}
+    <li>
+      <a href="{{ url }}">{{ name }}</a>
+    </li>
+    {% endfor %}
+  </ul>
+
+  {{ page_footer(
+    secondary_link=url_for('.api_integration', service_id=current_service.id),
+    secondary_link_text='Back to API integration'
+  ) }}
+
+{% endblock %}