Improve the Getting Started experience for platforms

The attached .pdf file contains some improvement ideas how the Getting Started experience of platforms could be improved on the Qt documentation's Getting Started page (https://doc.qt.io/qt-5/gettingstarted.html ). Shortly:

• Platform Requirements (https://doc.qt.io/qt-5/gettingstarted.html#platform-requirements) should cover all platforms and provide links to pages that contains instructions how to set up the environment and how to test it quickly
• list of Qt examples should be updated with the platform specific examples
  • https://doc.qt.io/qt-5/all-examples.html
  • https://doc.qt.io/qt-5/qtexamplesandtutorials.html
• Create Your First Applications (https://doc.qt.io/qt-5/gettingstarted.html#create-your-first-applications) topic in Getting Started page could be clarified
  • topic does not provide platform specific information and it's ok for me --> documentation text should emphasize this too

Task was created as a result of spike QTBUG-84116.