Thank you @BillRaymond. I didn’t actually create anything – I forked an existing template. I will work my way through the site you mentioned.
I think I have a plan of action though. From the git commits, it seems that this template worked in 3.x, so I can create a dev branch while I work through making it work in 4.x and do a separate 3.latest install while I work on the content. I will ask how to do this in a separate thread.
edit Add: and of course the separate thread already exists: Install a specific version of jekyll.