platform/wmi: Update driver development guide

platform/wmi: Update driver development guide

New WMI drivers should use the new buffer-based WMI API instead of
the deprecated ACPI-based API. Update the driver development guide
to recommend the buffer-based API to driver developers and explain
the purpose of struct wmi_buffer.

Also update the ACPI interface documentation to describe the
conversion rules for converting ACPI objects into WMI buffers.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://patch.msgid.link/20260116204116.4030-10-W_Armin@gmx.de
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>