Improve karpenter-convert input format requirement

[hybby]

Description

How can the docs be improved?
Posting this mostly just to help other people who stumble up against "error: no objects passed to convert" when using karpenter-convert :)

The karpenter-convert tool is great - but from the usage information (and the v1beta1 upgrade docs, it's not clear what the required input format for AWSNodeTemplate and Provisioners should be.

I found myself butting up against an error in the tool by trying what I thought made sense while following the docs:

# my inferral
❯ k get awsnodetemplate -o yaml > awsnodetemplate.yaml

# commands in docs (https://karpenter.sh/v0.32/upgrading/v1beta1-migration/#upgrade-procedure)
❯ CLUSTER_NAME=mycluster
❯ export KARPENTER_NODE_ROLE="KarpenterNodeRole-${CLUSTER_NAME}"
❯ karpenter-convert -f awsnodetemplate.yaml | envsubst
error: no objects passed to convert

It took a bit of diving through the code to find out that the tool didn't like lists of objects being passed, like -o yaml outputs:

❯ head awsnodetemplate.yaml
apiVersion: v1
items:
- apiVersion: karpenter.k8s.aws/v1alpha1
  kind: AWSNodeTemplate

Instead, the tool wanted flattened manifests - I grabbed each one individually and processed each file using the directory processing feature of the tool:

❯ for awsnodetemplate in $(kubectl get awsnodetemplates --no-headers | awk '{print $1}') ; do kubectl get awsnodetemplate ${awsnodetemplate} -o yaml > ${awsnodetemplate}.yaml ; done

❯ head foo.yaml
apiVersion: karpenter.k8s.aws/v1alpha1
kind: AWSNodeTemplate
...

❯ karpenter-convert -f foo.yaml
apiVersion: karpenter.k8s.aws/v1beta1
kind: EC2NodeClass

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[engedaam]

@hybby are you will to add this guidance as part of the does for v0.32?