feat(reference): Fill out reference page index (#98)

_data/navigation.yml

@@ -148,6 +148,8 @@ reference:
       url: /reference/api/swagger-ui/
   - title: Cloud Providers
     children:
+      - title: Overview
+        url: /reference/providers/
       - title: Amazon Web Services
         url: /reference/providers/aws/
       - title: Azure

reference/index.md

@@ -5,4 +5,6 @@ sidebar:
   nav: reference
 ---
 
-{% include toc %}
+Welcome to the Spinnaker Reference pages. These pages are intended
+for advanced Spinnaker users and developers looking for detailed explanations
+of how the various parts of Spinnaker work and fit together.

reference/providers/index.md

@@ -1,8 +1,27 @@
 ---
 layout: single
-title:  "Cloud Providers"
+title:  "Overview"
 sidebar:
   nav: reference
 ---
 
-links to providers
+{% include toc %}
+
+Once you're familar with the definition and setup of a [Cloud
+Provider](/setup/providers/), you can read these reference pages for
+information on how Spinnaker concepts map to those of the Cloud Provider, as
+well as how the various Spinnaker operations are performed at the Cloud
+Provider level. 
+
+## Supported Providers
+
+These are the Cloud Providers currently supported by Spinnaker:
+
+* [App Engine](/reference/providers/)
+* [Amazon Web Services](/reference/providers/)
+* [Azure](/reference/providers/azure/)
+* [Google Compute Engine](/reference/providers/gce/)
+* [Kubernetes](/reference/providers/kubernetes/)
+* [Openstack](/reference/providers/openstack/)
+* [Oracle](/reference/providers/oracle/)
+* [Titus](/reference/providers/titus/)