Update documentation to reference NavigationBar

[guyluz11]

• BottomNavigationBar is deprecated in favor of NavigationBar #4 Moved doc to use NavigationBar (onTap -> onDestinationSelected, currentIndex -> selectedIndex ...)
• Fixed Spotlight example did not show any spotlight
• main.dart example now demonstrates FlipBox as well as Spotlight.
• Made naming in doc consistent, 2 related gifs used the terms nev and nevegation which is not consistent.
• Added browser support to the example

• Is there a way to use "spotlight Navigation Bar" as standelone widget  #1 Fixed spotlight location is incorrect when NavigationBar is smaller then screen size