fix: Add description of storageClassesName option for karmadactl configuration

[highpon]

What type of PR is this?

/kind documentation

What this PR does / why we need it: When etcd storage mode is PVC, storageClassesName is not empty. However, this is not written in Karmada docs. This PR adds description for storageClassesName.

Which issue(s) this PR fixes: Fixes # None

Special notes for your reviewer:

[karmada-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[karmada-bot]

Welcome @highpon! It looks like this is your first PR to karmada-io/website 🎉

[RainbowMango]

@highpon Thank you for doing this. Could you please take a look at the failing DCO check?