Add an index page for the API integration

This commit adds a placeholder page which, for now, just has links to
the API keys page and links to the clients.

There’s more stuff to come on this page, but this commit just does the
reorganising so that it’s easier to review.

app/templates/views/api/index.html

@@ -0,0 +1,32 @@
+{% extends "withnav_template.html" %}
+{% from "components/table.html" import list_table, field, hidden_field_heading %}
+{% from "components/api-key.html" import api_key %}
+
+{% block page_title %}
+  API integration – GOV.UK Notify
+{% endblock %}
+
+{% block maincolumn_content %}
+
+  <h1 class="heading-large">
+    API integration
+  </h1>
+
+  <a href="{{ url_for('.api_keys', service_id=current_service.id) }}">API keys</a>
+
+  <h2 class="heading-medium">API clients</h2>
+  <ul class="list list-bullet">
+    {% for name, url in [
+      ('Java', 'https://github.com/alphagov/notifications-java-client'),
+      ('Node JS', 'https://github.com/alphagov/notifications-node-client'),
+      ('PHP', 'https://github.com/alphagov/notifications-php-client'),
+      ('Python', 'https://quis.github.io/govuk_documentation_prototype/'),
+      ('Ruby', 'https://github.com/alphagov/notifications-ruby-client')
+    ] %}
+    <li>
+      <a href="{{ url }}">{{ name }}</a>
+    </li>
+    {% endfor %}
+  </ul>
+
+{% endblock %}