Add Qt Quick Controls 2 TreeView and Calendar documentation to doc.qt.io

We need the documentation of all Qt additional libraries in doc.qt.io, as at least now the documentation is the place to find information about additional packages. 

We will have three additional libraries in the marketplace:

• Qt Quick Calendar
• Qt Quick TreeView
• Qt Quick MultiEffect

Their documentation must be added to doc.qt.io, with the following considerations taken into account:

• These additional libraries should be clearly separated from Qt modules.
• It should be explicitly mentioned that they are not part of Qt.
• They are available under GPL and via Qt Marketplace under the Marketplace license.