Update docs to include EKS cluster creation permissions

```
Error: creating EKS Cluster (cwagent-eks-integ-742c94a460e0d907): AccessDeniedException: User: [ARN] is not authorized to perform: eks:CreateCluster on resource: arn:aws:eks:us-west-2:***:cluster/cwagent-eks-integ-742c94a460e0d907
```

The public docs say to attach the `AmazonEKSClusterPolicy`, but I didn't see that you must also create an EKS cluster IAM role: https://docs.aws.amazon.com/eks/latest/userguide/service_IAM_role.html#create-service-role

I'm not sure if we really care to create a whole new role, and it sucks that there isn't a managed policy for this. We should include the necessary permissions in the existing set up documentation

Start with the existing setup documentation and the linked Amazon EKS service IAM role documentation. Clarify the EKS cluster creation prerequisites, including whether a separate cluster IAM role is required and which permissions must be documented; done means the setup instructions cover the permissions that prevent the reported AccessDeniedException.

Written by the indexing model from the issue text.