So after a lot of floundering about, where I tried various things, I managed to import as follows:
*= require font-awesome
Is this fine or might I expect other problems by doing it this way?
Also, while I admit I’m fairly new to Rails - and possibly not that bright - but is the whole “assets pipeline” business really supposed to be this complicated and problematic to use?
(I also had problems earlier when I was following Hartl’s Rails tutorial, and trying to import bootstrap his way, instead of with the -c bootstrap flag was actually triggering a segmentation fault on my MacOS system.)
After a fair amount of trial and error and Googling it dawned on me that since building an app with the -c bootstrap flag introduces the Yarn package manager into the equation, that is how you are supposed to add new front end libraries.
From the command line you can type yarn add @fortawesome/fontawesome-free, as it says here (not sure if that “fortawesome” is a typo or what, but it’s copied straight from the page), and then in the application.bootstrap.css file you should be able to do your @import.