docs: Add note to sys_buttons about using InputController instead

docs/api/sys_buttons.rst

 ``sys_buttons`` module
 ======================
 
+.. note::
+
+   This is a rather low-level way to access buttons, consider using `InputController` instead:
+
+   https://docs.flow3r.garden/badge/programming.html#example-1d-automatic-input-processing
+
 .. automodule:: sys_buttons
    :members:
    :undoc-members: