Mention API extensibility as a front-page feature

Kubernetes has really good options for extending and customizing its
behavior. Mention these right on the front page.

Author: Tim Bannister

content/en/docs/concepts/extend-kubernetes/_index.md

@@ -7,6 +7,10 @@ reviewers:
 - lavalamp
 - cheftako
 - chenopis
+feature:
+  title: Designed for extensibility
+  description: >
+    Add features to your Kubernetes cluster without changing upstream source code.
 content_type: concept
 no_list: true
 ---