Top-level page for describing API changes between Qt5.x -> Qt6.0

Something like this is needed:
https://doc.qt.io/qt-5.9/portingguide.html
The linked page describes Qt 4 -> Qt 5, we need the same for Qt 5 -> Qt 6.

If the documentation team can provide the basic structure, it is more likely
that devs will start adding stuff to it early.

Gatis.