Doc: Refactored and focused the Qt QML documentation.

Doc: Refactored and focused the Qt QML documentation.

Before it wasn't clear to what the module provided, especially
with the coupling of Qt QML and Qt Quick.

The doc is centered around these documentation:
-QML refernce: the language syntax and the application constructs
-JavaScript environment: the environment provided by the module
-Integration with C++: more about the engine and the C++ API
-QML Types and Classes reference (created by \qmlmodule and \module)

The distinction are made in the directory and the section titles in
the Qt QML landing page.

Change-Id: I033bfcbf8368b94ffa5ee4b1225bee74347f53eb
Reviewed-by: Martin Smith <martin.smith@digia.com>