Improve documentation for using Studio Components on airgapped/offline machines

We should document the following:

• How Qt Design Studio projects need Git, and Internet connection, and SSL access by default (QDS-9623)
• What BUILD_QDS_COMPONENTS is for, and how to use it on offline machines
• Clearer instructions at https://doc.qt.io/qtdesignstudio/quick-converting-ui-projects.html#adding-qt-quick-designer-components-to-qt-installations
  • It should use qt-cmake instead of manually specifying CMAKE_INSTALL_PREFIX
  • Building projects offline should be mentioned as a use-case/reason for manually installing Qt Quick Studio Components