Add a Qt example "Web" category

Qt WebEngine examples, Qt WebEngine Widgets examples, and Qt WebView examples arguably do not have a good category in Qt Examples and Tutorials. Some of them might be good Desktop or Embedded examples, but from the technology stack point of view, no other category fits really well.

I suggest therefore creating a 'Web' or 'Web and Cloud' category, which would collect examples from above modules, and maybe Qt WebChannel examples (which also fit the Networking category, but is typically used together one of the other Qt Web* modules).