doc: Mention terraform-aws-modules in cluster explanation.

As part of v5 migration, aws-ia#1428
removes "Node Groups" page.

As a result, new users likely lost what to do when configuring a cluster.

We at least need to mention that we use `terraform-aws-modules/eks/aws`
to avoid confusion.

docs/core-concepts.md

@@ -12,7 +12,9 @@ This document provides a high level overview of the Core Concepts that are embed
 
 ## Cluster
 
-A `cluster` is simply an EKS cluster. EKS Blueprints provides for customizing the compute options you leverage with your `clusters`. The framework currently supports `EC2`, `Fargate` and `BottleRocket` instances. It also supports managed and self-managed node groups. To specify the type of compute you want to use for your `cluster`, you use the `managed_node_groups`, `self_managed_nodegroups`, or `fargate_profiles` variables.
+A `cluster` is simply an EKS cluster. EKS Blueprints provides for customizing the compute options you leverage with your `clusters`. The framework currently supports `EC2`, `Fargate` and `BottleRocket` instances. It also supports managed and self-managed node groups.
+
+We rely on [`terraform-aws-modules/eks/aws`](https://registry.terraform.io/modules/terraform-aws-modules/eks/aws/latest) to configure `clusters`. See our [examples](getting-started.md) to see how `terraform-aws-modules/eks/aws` is configured for EKS Blueprints.
 
 ## Add-on