Improve Qt CMake Documentation

Now that CMake is becoming a major player in Qt, we should try to improve the current CMake Manual that is part of the Qt Reference documentation: http://doc.qt.io/qt-5/cmake-manual.html

We have the following options:

• Improve the manual as a part of Qt docs by restructuring it and adding information
• Add documentation about Qt-specific features upstream to CMake documentation:  https://cmake.org/cmake/help/latest/index.html 

There is already a Qt-specific section in the CMake docs that contains a link to the Qt CMake Manual: https://cmake.org/cmake/help/latest/manual/cmake-qt.7.html 

CMake documentation is already shipped as a Qt Help file, so we should look into delivering as a part of Qt docs.

If we want to keep the CMake Manual in the Qt docs, I would suggest the following changes:

• Move all the sections to separate pages and create a front page with mostly a TOC;
  • Getting Started
  • Building Common Project Types
    • Shared Library
    • Static Library
    • Unit Tests for Libraries
    • Widget-Based Applications (that use libraries)
    • Qt Quick Applications (that use libraries)
    • Console Application
    • Applications with Translations
    • Distributables
  • Reference
    • Variables
    • Macros

Each variable and macro should be listed on its own page, with standard content and examples of its use. Unfortunately, the CMake docs are not the best example I've seen of such documentation: https://cmake.org/cmake/help/latest/variable/CMAKE_INCLUDE_CURRENT_DIR.html 

• Add links to the CMake docs

If the CMake Manual stays in the Qt docs, should the sources also stay in the qtdoc module, or should they be moved closer to the CMake code?