Update the structure of the QtCore module to follow the diataxis framework

The current structure of the Qt Core will be evaluated

A new structure will be proposed based on the diataxis framework.

The documentation team will review the proposed structure and give their opinions

A final structure will be decided on.

Table of contents for QtCore: https://doc-snapshots.qt.io/qt6-dev/qtcore-toc.html

Remove: Java-style iterator 

Add: Dates, Times, Time Zones and Calendars.  Also one for Localization in the TOC.

Miro board for discussion: https://miro.com/app/board/uXjVLdRH_Nc=/