Provide ability to inject custom scripts into template

[jroper]

I'd like the ability to be able to inject custom scripts into the template, without having to fork the template. I'm not sure how/whether Antora supports this (if it does at all).

My use case is in Cloudstate, when we used Paradox we had some JavaScript that looked up the current released version of the CLI from our CLI downloads server, and automatically updates references to it in our pages so that our pages always pointed to the latest version of the CLI, rather than us having to manually update them. I'd like to be able to include this script in the Lightbend UI theme for the Cloudstate docs - in the footer so that it doesn't slow initial render of the page down.