Explain the role and use of ICU and other backends for Unicode in Qt

Due to the fact that Qt moved to use Unicode, the role and use of ICU (https://en.wikipedia.org/wiki/International_Components_for_Unicode) are now more important for users. For example, the QTBUG-114136 has been originally reported to support GB 18030-2022 directly in Qt, since the report originally did not know that it could be brought directly via ICU. 

Googling in Qt docs shows that the use of ICU in Qt is not explained. Likewise, it is not clear which other backends are used for cases when ICU is not used to build Qt. The same search for Unicode points to https://doc.qt.io/qt-6/unicode.html which is not mentioning or referring to ICU.  

I wish users would better informed about all this in the Qt docs