For user docs, yelp is the standard viewer for all GTK desktops. It has a hidden feature where it can open URIs like man:uscan or info:cp (ironically Help doesn't provide documentation for itself). yelp is a viewer for documentation with a base file of index.docbook (for docs written in DocBook) or the GNOME 3.0 era index.page (for docs written in Mallard or the Ducktype variant). I count about 186 of these files on my system for the "C" language (English).
I know very little about KDE, but a quick look at their documentation [1] points out that khelpcenter looks for HTML documentation listed in .desktop files. I count about 168 packages participating [2]. They appear to nearly all be apps for KDE. Therefore, if we proceed with my recommendation, we could mention as alternatives: the .devhelp2 format for API documentation written in HTML for users of GTK desktops. For user docs, we could link to both the KDE format [1] and Mallard [3]. [1] https://invent.kde.org/system/khelpcenter/-/blob/master/README.metadata [2] https://codesearch.debian.net/search?q=path%3A.desktop+X-DocPath [3] https://web.archive.org/web/20250117135803/http://projectmallard.org/ Thank you, Jeremy Bícha
