Test, document and explain the use of additional fonts with Qt WebEngine

In the issue QTBUG-133901, a user reported a crash in QWE while trying to load and use an additional font in HTML (with CSS) which was previously loaded via Qt API (QFontDatabase::addApplicationFont) in the app code before instantiating QWE. The reason for this is that Chromium as a part of QWE does not have proper access to the fonts loaded in the Qt's space.

Despite, that this should not cause a crash, but rather an error if it is not supported, it also seems that this kind of use case is not documented in QWE. It is also not clear how QWE interacts with platform services to list and load additional fonts.

Some work should be done to test, document, and explain these use cases.