Documentation to clarify installation of downloaded packages

[SealedServant]

As mentioned in pdm-project/pdm#2828

The documentation could clarify how to properly install packages fetched with pdm download command using the pdm install command.

[znicholls]

A +1 here from me. I have done pip installs using local wheelhouses before, but it isn't clear to me how to use the output from pdm download within the overall pdm workflow. A quick explanation/docs/pointer here would be much appreciated!