Replace URL reference in Shoveler param doc

[brianhlin]

Remove the URL reference from https://docs.pelicanplatform.org/parameters#Shoveler as it's confusing for users (I don't think users need to follow this doc to install the shoveler in Pelican). I'd replace it with a summary blurb:

This shoveler gathers UDP monitoring messages from XRootD servers and sends them to a reliable message bus.

And maybe add the link to the project https://github.com/opensciencegrid/xrootd-monitoring-shoveler