Add documentation on how to use a module from CMake, alongside
the existing documentation about qmake. Separate generic info
from module-specific examples, to make it possible to use one
include file in all modules.
While at it, also remove the mentioning of the central include;
it is not something we should actively advocate anymore. Instead,
the documentation of every class gives the correct include to use.
Task-number: QTBUG-73058
Change-Id: I6b3c0e5ea218dd9c06a491c8fb799a7fcf42dd92
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
