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

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

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

boards/dptechnics/walter/doc/index.rst

@@ -1,7 +1,4 @@
-.. _walter:
-
-DPTechnics Walter
-#################
+.. zephyr:board:: walter
 
 Overview
 ********
@@ -11,12 +8,6 @@ with a Sequans Monarch 2 GM02SP LTE-M/NB-IoT/GNSS modem.
 More information about Walter can be found on the `QuickSpot Website`_ and on the
 `QuickSpot GitHub page`_.
 
-.. figure:: img/walter.webp
-   :align: center
-   :alt: DPTechnics Walter board
-
-   DPTechnics Walter board (Credit: DPTechnics bv)
-
 Hardware
 ********