[Documentation] Update and improvements of Buttons documentation.

Documentation about MaterialButton and MaterialButtonToggleGroup are now in the same doc.

PiperOrigin-RevId: 292420210

Author: leticiarossi

docs/components/Buttons.md

@@ -160,4 +160,4 @@ shrinks the FAB to show just the icon.
 ## Related Concepts
 
 -   [FloatingActionButton](FloatingActionButton.md)
--   [MaterialButton](MaterialButton.md)
+-   [Buttons](Buttons.md)