Add pages for create/view/revoke API keys

Copying what they’ve done on GOV.UK Pay, we should let users:
- generate as many keys as they want
- only see the key at time of creation
- give keys a name
- revoke any key at any time (this should be a one way operation)

And based on discussions with @minglis and @servingUpAces, the keys should be
used in conjunction with some kind of service ID, which gets encrypted with the
key. In other words the secret itself never gets sent over the wire.

This commit adds the UI (but not the underlying API integration) for doing the
above.

app/templates/views/documentation.html

@@ -0,0 +1,49 @@
+{% extends "withnav_template.html" %}
+{% from "components/page-footer.html" import page_footer %}
+{% from "components/api-key.html" import api_key %}
+
+{% block page_title %}
+  GOV.UK Notify | API keys and documentation
+{% endblock %}
+
+{% block maincolumn_content %}
+
+    <div class="grid-row">
+      <div class="column-two-thirds">
+
+        <h1 class="heading-xlarge">
+          Developer documentation
+        </h1>
+
+        <h2 class="heading-medium">
+          How to integrate GOV.UK Notify into your service
+        </h2>
+
+        <p>
+          blah blah blah this is where we tell you how the API works
+        </p>
+
+        <h2 class="heading-medium">Repositories</h2>
+
+        <p>
+          <a href="https://github.com/alphagov/notifications-api">GOV.UK Notify API</a>
+        </p>
+
+        <p>
+          <a href="https://github.com/alphagov/notify-api-client">GOV.UK Notify Python client</a>
+        </p>
+
+        <h2 class="heading-medium">API endpoint</h2>
+
+        <p>
+          https://www.notify.works/api/endpoint
+        </p>
+
+        <p>
+          <a href="{{ url_for('.api_keys', service_id=service_id) }}">API keys for your service</a>
+        </p>
+
+      </div>
+    </div>
+
+{% endblock %}