CMake: Provide public API for building documentation with qdoc

With qmake it's possible to use the "make docs" (and related) targets to call qdoc with the correct parameters to generate documentation (in various formats).

In the context of QTBUG-73351 as well as the popularity of cmake among the users of Qt, it is desirable to add similar functionality to the cmake integration in Qt.

This could be implemented as a function (qt_build_docs or so) to the CMake API that Qt provides (similar to the functions for RCC handling, etc.) in Qt 5 and be re-used in the setup of building Qt with cmake itself.

Similar to qmake, this could generate an extra target that users can call (in cmake terms using add_custom_target).

Providing this functionality already in Qt 5 would serve our existing users well who would like to use qdoc for the documentation of their own projects while using cmake as a build system.

It is desirable to provide this function in Qt itself instead of just suggesting users to call qdoc itself, given the complexity of parameters that need to be provided with qdoc – especially with regards to the correct compiler flags. There is information within the build system that is essential to a successful qdoc invocation that justifies Qt providing the integration.