Add docs about the new cluster app

I think it makes sense to mention the new cluster app in the Neptune docs, in the section https://doc-snapshots.qt.io/neptune3ui-dev/neptune3ui-overview.html#instrument-cluster as a subsection. This should tell:

• it is an app which can run independently from Neptune
• due to this it has it own life cycle and does not depend on System UI
• as a stand-alone app it can run on a separate board or in a dedicated hypervisor domain and connect to backends via a network connection
• it shares the UI design and generic components with the Neptune 3 UI
• compared to the cluster app in Neptune 3 UI, it accepts touch and mouse events to switch between modes in the content area: vehicle view, media view and map view