Update docs to mention the JFrog CLI build-discard command

[Minreaux]

Is your feature request related to a problem? Please describe.

I did not know how to handle discarding Artifactory builds (and their artifacts) once they have been uploaded by Jenkins. I also did not know how to handle Jenkins builds marked with the "Keep Forever" option on the Artifactory side.

I struggled with this until I eventually found the jf rt build-discard command. This command "completes the loop" of using the Jenkins JFrog plugin with Artifactory.

For example:

1. jf rt upload to upload Artifacts
2. jf rt build-publish to publish the build-info object
3. jf rt build-discard to manage the lifetime of the builds and their corresponding artifacts in Artifactory

Describe the solution you'd like to see

I think it would be very useful for new JFrog users like myself who are trying to setup Artifactory with Jenkins for the first time if the Jenkins JFrog CLI docs called out the jf rt build-discard command since it is needed to "complete the loop" for Jenkins/Artifactory builds/artifacts integration.

Describe alternatives you've considered

N/A

Additional context

N/A