Doc: Link CMake variable and property pages to the new how-to page

Add a link to "Setting CMake variables and properties" to every
documented CMake variable and property page. That page explains how to
set variables and properties in a project, on the command line, and in
IDEs.

Task-number: QTBUG-122379
Pick-to: 6.12
Change-Id: If91a73527d7c77f8938f235114840bbc00d0e9f0
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>