Lack of documentation for validating karmada images with cosign

[zishen]

What documentation would you like to be added:
Documentation for validating karmada images with cosign.

Why do you think this document is needed:
Document completion for Part3 of karmada-io/karmada#3435

Where do you think the document should be placed:
I think the document could be placed under /Documentation/administrator/verify-artifacts.md

[zishen]

/assign @zishen

[RainbowMango]

This duplicates with #379.