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-kubectl/kubectl-plugins.md

@@ -3,7 +3,7 @@ title: Extend kubectl with plugins
 reviewers:
 - juanvallejo
 - soltysh
-description: With kubectl plugins, you can extend the functionality of the kubectl command by adding new subcommands.
+description: Extend kubectl by creating and installing kubectl plugins.
 content_type: task
 ---