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/tls/_index.md

@@ -1,5 +1,6 @@
 ---
 title: "TLS"
 weight: 100
+description: Understand how to protect traffic within your cluster using Transport Layer Security (TLS).
 ---