Issue with platform JSON for Qt Creator integration of Qul Platform

Issue with documentation reported by a customer trying to perform a custom platform port. 

Link to document with the issue: https://doc.qt.io/QtForMCUs/platform-porting-guide-getting-started.html

The documentation suggests that qulVersion can be left as "@CMAKE_PROJECT_VERSION@". However according to the customer, leaving this value as default causes an issue on kit creation within the IDE that causes Qul::Platform to be missing at build time.

Looking at the documentation it does seem easy to come to the conclusion that qulVersion can be left as default. 

The customer suggested some improvements which might be able to help prevent this issue in the future. 

1. Calling out how specifically how “@CMAKE_PROJECT_VERSION” is set
2. Indicating that the kit will not function correctly if qulVersion is not set