Update docs to mention the JFrog CLI build-discard command

### 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

Start by locating the Jenkins JFrog CLI documentation and reviewing how it currently describes uploading artifacts and publishing build information. Add the jf rt build-discard command to the documented Jenkins/Artifactory workflow, including its role in managing build and artifact lifetimes; done means new users can find this step alongside the existing commands.

Written by the indexing model from the issue text.