Include online installer CLI download command

https://doc.qt.io/qt-6/get-and-install-qt.html

https://doc.qt.io/qt-6/get-and-install-qt-cli.html#new-installation-with-qt-online-installer

Upon attempting to get Qt installed on a headless machine, I needed to figure out how to get the Qt online installer executable onto my machine. This proved to be more elusive than I was anticipating. Ultimately, the below wget command worked for me, but it seems like it would provide a benefit to everyone who faces this obstacle to have this type of command readily available on one of the two above linked documentation pages. As far as I could tell this is not anywhere in the documentation. It just assumes you already have the installer downloaded.

wget
https://download.qt.io/official_releases/online_installers/qt-online-installer-linux-x64-online.run