Qt Writing Guidelines: Consolidate building instructions

We no longer recommend building with make, so shift the focus in the Qt 6 wiki pages to ninja/cmake.

Direct links to https://wiki.qt.io/Building_Qt_Documentation which is actively maintained.

Audience:

A) People who want to create a documentation set that is based on Qt.

B) People who want to create a documentation set not based on Qt. A generic documentation set.

KDE would probably fall into B. While a Qt tool/module documentation set would belong to A.