Fix opening subpages of documentation

When opening a URL like help:/kcalc we don't resolve relative URLs correctly, e.g. simple-mode.html resolves to help:/simple-mode.html instead of help:/kcalc/simple-mode.html

This doesn't happen when opening help:/kcalc/ (i.e. with a trailing slash)

BUG: 484176
(cherry picked from commit 22f175cf)