Update Qt Widgets Notepad tutorial

Since Qt Creator 6.0, the "File > New File or Project" menu item has been split into "New Project" and "New File". The contents of the wizard template and the generated code have also change. CMake is used by default, which means we should explain CMakeLists.txt instead of notepad.pro.

I want to update the topic because it opens from Qt Creator Welcome mode, Tutorials tab, and it should match the Qt Creator version as closely as possible.

https://doc.qt.io/qt-6/qtwidgets-tutorials-notepad-example.html