Document qtbase Qt 6.2 CMake API

The following CMake functions provided by qtbase are missing documentation:

• qt_add_library Done
• qt_add_plugin Done
• qt_set_finalizer_mode (will be TP in 6.2)
• qt_disable_unicode_defines Done
• qt_allow_non_utf8_sources Done
• qt_extract_metatypes

The following need adjustments

• qt_import_plugins mentions Qt5 and is missing doc about NO_DEFAULT option Done
• qt_finalize_target says it's still in TP Done

https://doc-snapshots.qt.io/qt6-dev/cmake-command-reference.html

Furthemore https://doc-snapshots.qt.io/qt6-dev/cmake-get-started.html should be updated to mention the qt_ prefixed functions.