(core): improve the GUI documentation
Description
Currently, the SAD-GUI section lists the gui default services for view, menubar and menu and speak about backend
but does not explain how it works in the implementation.
Proposal
The SAD is the fw4spl documentation for experts, it may be useful to explain how the GUI is implemented. The current documentation of how to use the service should be in the tutorials documentation, may be we should add the doc for TutoGui.