Update documentation for CNI plugin logging

[jdn5126]

What would you like to be added:
Clarify in https://github.com/aws/amazon-vpc-cni-k8s#aws_vpc_k8s_plugin_log_file that since CNI is exec'ed by the container-runtime in k8s 1.24+, setting the log file to stdout or stderr will redirect to stdout/stderr for container-runtime process (usually containerd), NOT aws-node pod.

Also, fix the logic in https://github.com/aws/amazon-vpc-cni-k8s/blob/master/pkg/utils/logger/zaplogger.go#L130 to handle the string "stderr".

Also, clarify that stderr is an accepted value for https://github.com/aws/amazon-vpc-cni-k8s#aws_vpc_k8s_cni_log_file and that stdout/stderr will show up in the aws-node pod logs since aws-k8s-agent (IPAMD) process runs in the pod.

Why is this needed:
To improve readability and reduce confusion for customers.

[jdn5126]

Also, clarify at https://github.com/aws/amazon-vpc-cni-k8s#aws_vpc_k8s_cni_log_file that /host/ is the container file system path, while /var/log/... is the node path.

Also, clarify that setting https://github.com/aws/amazon-vpc-cni-k8s#aws_vpc_k8s_plugin_log_file to the empty string redirects logs to stderr, and that comes from upstream k8s practices

[github-actions]

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