Support Copy n Paste of Command Line commands in Ref Guide

From a discussion started in SOLR-16602, the suggestion is to remove $ from the command line examples in the docs to make the cut'n'paste option in the Ref Guide work for those commands.

In doing more investigation, it turns out that in Antora, there is a formatting option where commands get some extra visual indications, and that the copy icon works the way you would expect. It will even handle not copying the output of a command, just the command itself!

I am attaching two screenshots of the before and after, and the formatting changes.