doc: explain differences/disadvantages of directory imports and module imports

In https://doc.qt.io/qt-6/qtqml-syntax-directoryimports.html, there should be an explanation of the difference between directory imports and QML module imports, and links to how to create QML modules (like the new page created in QTBUG-133547, for example).

The page misses e.g. that a directory import can't be used if C++ types are needed (potentially at a later point during development), and that you can mock modules when using module imports by putting the mock modules location to the front of the import path, which can't be done with directory imports.