docs: support build with Qt6
- `qcollectiongenerator' is deprecated in Qt5 and removed in Qt6.
- In Qt5 and Qt6,
qhelpgenerator' can process
.qhcp' just fine. - The output of Qt5's
qcollectiongenerator',
qhelpgenerator' and Qt6 `qhelpgenerator' is the same.
Let's switch to `qhelpgenerator' in order to support Qt6 build.