Add info about the Marketplace license to Qt documentation

Due to the launch of QtCharts, QtPDF, and QtDesign Studio Bridge as separate sellable modules in the Qt Marketplace, and already other modules in the pipe, we should adjust our Qt Documentation in a way, that our users understand that these particle modules are also available under the Marketplace License Agreement.

E.g. under the Qt Add-on section in the documentation https://doc.qt.io/qt-5/qtmodules.html there could be another subsection called "Add-ons available under Commercial Licenses, or GNU General Public License v3, or Marketplace License Agreement" where QtCharts and QtPDF are listed. 

 In addition, on the Qt Licensing page (https://doc.qt.io/qt-5/licensing.html) the MP LA could also be added/mention.