Add documentation about the location and usage about Qt Linguist

Our customer often ask for the standalone Qt Linguist tool, what translation agencies could use to localise strings. Agencies do not need to install whole Qt to use Linguist, so this is why Qt Linguist is available for Windows in https://download.qt.io/linguist_releases/. We should document, how Qt users can find the standalone version, mention it's for Windows only and compatible with Qt 6. A reference to existing User Manual https://doc.qt.io/qt-6/qtlinguist-index.html should be mentioned as well.