intra-xrefs converted to plain text reference provide bad UX

[ghost]

Initially reported by @vmoutoussamy on IRC.

When we build individual books from a set, intra-xrefs are converted to plain text.
Having to dig through the web site to find the guide referenced as plain text is not a great experience.

Maybe we could add a build parameter that defines a URL prefix for intra-xrefs? However, the prefix would likely have to be different for different books (book.gnomeuser is likely in a different directory than book.security)/different formats (we should reference PDFs from PDFs, 1-HTML from 1-HTML)...

[vmoutoussamy]

Just a bit of context about my report. IMHO, our techy users would prefer the single HTML version of our docs over the regular HTML one. But then they could face situation like this: section 2.2 on https://www.suse.com/documentation/sles-15/singlehtml/art_modules/art_modules.html the paragraph doesn't have any interesting content apart from the references to other docs, so the user basically have to find his way to the references by himself.