ManPageDocumentation: expand unsupported links on pages

file:// URL links don't work in Qt WebEngine because it forbids such
file system access. help:/ links don't work because the KDE help URL
scheme is not registered with Qt WebKit or Qt WebEngine.

Currently the HTML of each man page contains two stylesheet links with
the following URLs:
file:///usr/share/kio_docfilter/kio_docfilter.css
help:/kdoctools5-common/kde-default.css

The kio_docfilter style sheet forces white background color, affects the
appearance of #header (the top banner that is hidden by
manpagedocumentation.css) and #footer (nicely separates the bottom
"Generated by kio_man version 22.04.2" text from the man page itself).

The kde-default.css affects many elements of a man page. Whether the
page looks better as a result is subjective. But at least it works as
intended and is consistent with KHelpCenter's rendering.