Improve top-level index page for documentation

- Adds Menus to the front page so that we
can actually find essential types like Menu and Action.
- Added a basic example for Action
- I also added ExclusiveGroup to the Controls
category to make it accessible.

Change-Id: Ife980b786870d2bf07d709e024cb97396870e6c7
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>