Update public documentation on how to build Qt from source

We should update the official docs on how to build Qt from source, specifically the pages at

https://doc-snapshots.qt.io/qt6-dev/build-sources.html
https://doc.qt.io/qt-5/build-sources.html

Currently, we have docs on how to build Qt with CMake at the following locations:
https://code.qt.io/cgit/qt/qtbase.git/tree/cmake/README.md
https://wiki.qt.io/Building_Qt_6_from_Git ( counterpart Qt5 https://wiki.qt.io/Building_Qt_5_from_Git)
https://www.qt.io/blog/qt-6-build-system

We should update the official docs (that are found at https://code.qt.io/cgit/qt/qtdoc.git/tree/doc/src/getting-started/installation.qdoc ) to mention Qt 6, and update the steps appropriately.

In a recent CMake meeting, we decided we should recommend usage of the `configure` and `qt-configure-module` scripts as the official way to build Qt. So we shouldn't mention using the qt-cmake-private script or the cmake executable directly.