CMake: Improve CMake template in manual

Include notes about how to work with .ui files using CMake's AUTOUIC feature.
Use CMake 3.1 as minimum required CMake version as this is required
since 5.10 anyway. Also note that CMAKE_AUTOUIC requires CMake >= v3.0.0.

Task-number: QTBUG-59202
Task-number: QTBUG-63519
Change-Id: I0a0814c6dda6746f399b1c883932751e44993ebc
Reviewed-by: Kevin Funk <kevin.funk@kdab.com>