Add descriptions to Concept sections

Each section directly beneath Concepts gains a description.

content/en/docs/concepts/overview/_index.md

@@ -1,4 +1,5 @@
 ---
 title: "Overview"
 weight: 20
----
+description: Get a high-level outline of Kubernetes and the components it is built from.
+---

content/en/docs/concepts/overview/components.md

@@ -3,6 +3,9 @@ reviewers:
 - lavalamp
 title: Kubernetes Components
 content_type: concept
+description: >
+  A Kubernetes cluster consists of the components that represent the control plane
+  and a set of machines called nodes.
 weight: 20
 card: 
   name: concepts

content/en/docs/concepts/overview/kubernetes-api.md

@@ -4,6 +4,9 @@ reviewers:
 title: The Kubernetes API
 content_type: concept
 weight: 30
+description: >
+  The Kubernetes API lets you query and manipulate the state of objects in Kubernetes.
+  The core of Kubernetes' control plane is the API server and the HTTP API that it exposes. Users, the different parts of your cluster, and external components all communicate with one another through the API server.
 card:
   name: concepts
   weight: 30

content/en/docs/concepts/overview/working-with-objects/_index.md

@@ -1,5 +1,7 @@
 ---
 title: "Working with Kubernetes Objects"
 weight: 40
+description: >
+  Kubernetes objects are persistent entities in the Kubernetes system. Kubernetes uses these entities to represent the state of your cluster.
+  Learn about the Kubernetes object model and how to work with these objects.
 ---
-