Doc: Update Licensing documentation

Update licensing documentation overview, and reflect changes to Qt
licensing in 5.7.

  - Add GPLv3-licensed add-ons to the table
  - Document three.js license for Qt Canvas 3D
  - Document assimp license for Qt 3D
  - Link to Qt WebEngine documentation on Chromium 3rd-party licenses
  - Remove Qt WebKit licensing information
  - Add links to the top-level licensing page from the subpages

Task-number: QTBUG-51805
Change-Id: I589bd813fac728e8fd2894f7bc92702b53f5692f
Reviewed-by: Sami Makkonen <sami.makkonen@theqtcompany.com>
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>