boards: witte: adopt new zephyr:board directive and role

This updates the documentation of all the Witte boards to use
the new `zephyr:board::` directive.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

boards/witte/linum/doc/index.rst

@@ -1,7 +1,4 @@
-.. _linum:
-
-Witte Technology Linum Board
-############################
+.. zephyr:board:: linum
 
 Overview
 ********
@@ -11,10 +8,6 @@ touchscreen and another for access to other peripherals of microcontroller. Also
 of communications interfaces like UART with RS232 and RS485 capabillities, CAN bus compatible to
 FD standard, and networking over Ethernet.
 
-.. image:: img/linum-stm32h753bi-top.jpg
-     :align: center
-     :alt: Linum development board
-
 Hardware
 ********