Update docs to include artifact 'Actions' drop down descriptions

I couldn't find a place where we talk about the Actions drop down in the Harbor interface and how a user can take advantage of those options.

We should update the docs with screenshots to include information on the Copy Digest, Add Labels, Copy Artifact, and Delete options.

Important info to mention
* From [this comment](https://github.com/goharbor/website/pull/301#issuecomment-1073536713) We should mention that the Copy Artifact option will also copy any associated cosign signature accessories in addition to the artifact