Reorganize ResourceOp samples

[elikatsis]

Move the samples to different directories for the users to easier navigate to the desired usage demonstration.

Also add READMEs under samples/volumes/ and samples/snapshots/ with some documentation.

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This change is 

[elikatsis]

Hey @gaoning777, any opinion on this PR?

[gaoning777]

Hey, Ilias, thanks for your contribution. I'm trying to collect some teamwide feedback in terms of the samples and sample test plans during the past two days. I will respond soon. Thanks

[elikatsis]

/retest

[elikatsis]

Hi there!

Any update on this?
It could supplement the work I'll be doing regarding the documentation of ResourceOps and the rest of the entities (kubeflow/website#757, kubeflow/website#758).

Thanks!

[Ark-kun]

/lgtm

[Ark-kun]

/cc @gaoning777

[gaoning777]

/approve

[k8s-ci-robot]

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