Add descriptions for tasks

Add description metadata for each task section (English localization).
For sections that only have one page, make sure that that sole page has
description metadata set.

This change helps prepare for the migration to the Docsy theme.

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

@@ -1,5 +1,6 @@
 ---
 title: "Extend Kubernetes"
+description: Understand advanced ways to adapt your Kubernetes cluster to the needs of your work environment.
 weight: 90
 ---