Move kubectl overview to be section index

Also:
- use glossary definition in page introduction
- tidy broken link in What's Next section
- update links to refer to moved page

content/en/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions.md

@@ -1125,7 +1125,7 @@ with `foo` pruned and defaulted because the field is non-nullable, `bar` maintai
 
 CustomResourceDefinition [OpenAPI v3 validation schemas](#validation) which are [structural](#specifying-a-structural-schema) and [enable pruning](#field-pruning) are published as part of the [OpenAPI v2 spec](/docs/concepts/overview/kubernetes-api/#openapi-and-swagger-definitions) from Kubernetes API server.
 
-The [kubectl](/docs/reference/kubectl/overview) command-line tool consumes the published schema to perform client-side validation (`kubectl create` and `kubectl apply`), schema explanation (`kubectl explain`) on custom resources. The published schema can be consumed for other purposes as well, like client generation or documentation.
+The [kubectl](/docs/reference/kubectl/) command-line tool consumes the published schema to perform client-side validation (`kubectl create` and `kubectl apply`), schema explanation (`kubectl explain`) on custom resources. The published schema can be consumed for other purposes as well, like client generation or documentation.
 
 The OpenAPI v3 validation schema is converted to OpenAPI v2 schema, and
 show up in `definitions` and `paths` fields in the [OpenAPI v2 spec](/docs/concepts/overview/kubernetes-api/#openapi-and-swagger-definitions).