Add YAML file explanations: Introduction section

[Setup](https://www.eksworkshop.com/docs/introduction/setup/) > [In your AWS account](https://www.eksworkshop.com/docs/introduction/setup/your-account/) > Using eksctl
[Setup](https://www.eksworkshop.com/docs/introduction/setup/) > [In your AWS account](https://www.eksworkshop.com/docs/introduction/setup/your-account/) > Using Terraform

Start by reading the linked Introduction > Setup > In your AWS account pages for Using eksctl and Using Terraform. Identify the YAML files those pages use and document what each file contains and controls. Done means the introduction section explains the referenced YAML files for both setup paths.

Written by the indexing model from the issue text.