Use KDE_INSTALL_QTQCHDIR to install QCH documentation

...and don't hard code <prefix>/share/docs/qch. All other QCH files
on my system live in /usr/share/doc/qt5/. KDE_INSTALL_QTQCHDIR
(available since ECM 5.36.0) easily allows to customize this. It
defaults to DATAROOTDIR/doc/qch or the path reported by qmake(-qt5)
if KDE_INSTALL_USE_QT_SYS_PATHS is enabled.