Create new documentation structure

The documentation currently has no clear separation between Qt QML
and Qt Quick.  With recent commits like:
6c8378ea
and
ab1e5101


the separation between the language definition and implementation,
provided by Qt QML, and the standard library for the QML language,
provided by Qt Quick, is clear.

This commit creates a new documentation structure that is more
navigable and separates concepts into logical categories, with
clear separation between QtQML and QtQuick.  It also provides a more
generic QML Application Developer Resources page which contains links
to information for QML application developers.

Change-Id: Ia807ccfbfd24ffa0e1c7f0a51ed9d2ed3aa6a733
Reviewed-by: Martin Jones <martin.jones@nokia.com>