Add a QDoc command and CSS for background information

In tutorials, example documentation, and tools manuals, it would be useful to have a separate style for general background information for the task at hand. For example, the Qt Design Studio Manual now contains tutorials with "Learn Qt Quick" sections. It would be useful to somehow visually set apart those sections.

https://doc-snapshots.qt.io/qtdesignstudio-4.9/qt-design-studio-loginui1-example.html#learn-qt-quick-projects-and-files

The attached image displays a suggestion for the new style and icon by Diana de Sousa. The background color is the lightest Qt grey color and the icon is based on the academy hat icon used in Qt Creator to mark tutorials.