libhandy sample code

[colinkiama]

As well as there not being examples of how to use the widgets, there are differences in how things are handled compared to using the GTK 3 library e.g having to include Hdy.init () inside an Application's startup event. Stuff like this should be mentioned in more pages. Maybe every widget page?

I think that including sample code would really help make libhandy much easier to use in Vala. It would save so much time to compared to looking across repos for projects that have used libhandy in Vala.

Maybe we could also have images of the created widgets too like the Valadoc GTK 3 docs do (Widget Gallery) if that isn't too out of scope?