added AWS EKS cluster scaling/auto-scaling documentation for z2jh

[cdibble]

This is an unsolicited PR but I figured I'd add what I've learned about scaling on AWS EKS, since that was not yet included in the documentation for Expanding and contracting the size of your cluster.

[consideRatio]

Is this a valid step for anyone having an EKS cluster, no matter if it was setup with kops or eksctl initially? If not, I think it should be clarified that this is scoped specifically for AWS EKS clusters setup with eksctl.

[cdibble]

Good point. This is EKS specific and doesn't apply to either kops or a "manual" k8s installation in AWS. I'll clarify.

[cdibble]

I think it's clear now that this is EKS specific. You don't technically need to use eksctl to create the cluster (it can be done in the AWS console), but you have to install eksctl and configure it to point to your cluster. I've added that point as well.

[consideRatio]

Thanks you @cdibble!

[welcome]

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