Improve "Module Definition qmldir Files" page

Currently, the closest search result for "qml singleton" is this Qt for MCUs link:

https://doc.qt.io/QtForMCUs-2.1/qtul-qml-singleton.html

In addition to QTBUG-104546, we should break up https://doc.qt.io/qt-6/qtqml-modules-qmldir.html#contents-of-a-module-definition-qmldir-file into sections, rather than having it in a giant table. That would make it possible to give each bit of syntax its own section and make it easier to search for, and also make the text on the right column far more readable as it'd no longer be squished into such a small space.