Update and improve Qt 6 from Git instructions according to some findings

Learned how to build and test with Qt 6 following these

https://wiki.qt.io/Building_Qt_6_from_Git 

Collecting some improvements here to be reviewed before updating.

Findings (WIP):

• In "Configuring and Building"
  • To skip normal building and move to "Developer Builds" if testing is needed
• In sections where cmake is used
  • Make visible alternative approach to compile binaries with "ninja" command
• In sections where ctest is used
  • Make visible anternative approach to run test binaries directly
  • Running test binaries directly is only possibility to create i.e. testresults in XML or other format
  • Make visible that test binaries has help -option to show i.e. how to get testresults in XML format etc.