Document how to update Qt

Let's add a page giving an documentation overview how the user can update Qt.

1. Overview

Reasons for updating Qt

• Security Updates & Bug Fixes (-> Reference Security)
• New features
• Cross-link to https://doc.qt.io/qt-6/qt-releases.html

Updating Qt without recompilation

• Dynamic vs static builds (TODO: Do we define these somewhere already?)
• Toolchain requirements
• Cross-link to Source and Binary Compatibility (see also https://doc.qt.io/qt-6/qt-releases.html)
• Platform-specific signing requirements

Updating Qt in the Qt Online Installer

Updating Qt from other sources

• Yocto?

Recompiling Qt from Sources

• Applying Security Patches
• Cross-link to https://doc.qt.io/qt-6/build-sources.html