Add demo site

Add a new page for describing and linking to the new demo site.

Signed-off-by: Ben Kochie <superq@gmail.com>

Author: SuperQ

content/demo.md

@@ -0,0 +1,16 @@
+---
+title: Demo
+layout: page
+
+---
+# Demo Site
+
+There is a public demo site for Prometheus, Alertmanager, Grafana, and several exporters.
+
+[**View the demo**](https://demo.do.prometheus.io)
+
+The site is deployed with Ansible and the [Cloudalchemy Prometheus roles](https://github.com/cloudalchemy).
+
+The source to this demo site is [on Github](https://github.com/prometheus/demo-site).
+
+Special thanks to [DigitalOcean](https://digitalocean.com) for providing hosting resources.

layouts/header.html

@@ -69,6 +69,7 @@
             <li><a href="/docs/introduction/overview/">Docs</a></li>
             <li><a href="/download/">Download</a></li>
             <li><a href="/community/">Community</a></li>
+            <li><a href="/demo/">Demo</a></li>
             <li><a href="/blog/">Blog</a></li>
             <li><a href="https://github.com/prometheus"><i class="fa fa-github"></i></a></li>
             <li><a href="https://twitter.com/PrometheusIO"><i class="fa fa-twitter"></i></a></li>