docs: Add note to sys_buttons about using InputController instead

apparently sys_buttons is not ideal to use as it does not expose app button and OS button (users can swap them in settings, for ex. for single-hand left handed use) but instead directly exposes left and right shoulder buttons.

This is also the more visible option in the docs, as the other is buried in the programming page whereas sys_buttons is on the sidebar. I've added a note pointing devs to InputController documentation under Programming page to make it easier for app developers to use the right way to read the shoulder buttons.