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

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

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

boards/rakwireless/rak11720/doc/index.rst

@@ -1,7 +1,4 @@
-.. _rak11720:
-
-RAK11720
-########
+.. zephyr:board:: rak11720
 
 The RAK11720 is a WisBlock Core module for RAK WisBlock.
 It is based on the powerful ultra-low power Apollo3 Blue SoC (AMA3B1KK-KBR-B0)
@@ -13,10 +10,6 @@ comes in the same size and footprint as our RAK3172 module which gives
 you the opportunity to enhance your existing designs
 with BLE without designing a new PCB.
 
-.. image:: img/rak11720.webp
-   :align: center
-   :alt: RAK11720
-
 Hardware
 ********
 

boards/rakwireless/rak4631/doc/index.rst

@@ -1,7 +1,4 @@
-.. _rak4631_nrf52840:
-
-RAK4631
-#######
+.. zephyr:board:: rak4631
 
 Overview
 ********
@@ -16,10 +13,6 @@ the same TX power. This makes the RAK4631 an ultra-low
 power communication solution. RAK4631 can be comfortably
 programmed with ZephyrRTOS.
 
-.. image:: img/rak4631-front-parts.jpg
-     :align: center
-     :alt: RAK4631-NRF52840
-
 Hardware
 ********
 

boards/rakwireless/rak5010/doc/index.rst

@@ -1,4 +1,4 @@
-.. _rak5010_nrf52840:
+.. zephyr:board:: rak5010
 
 RAK5010
 #######
@@ -24,10 +24,6 @@ quick testing and prototyping tool for applications
 requiring NB-IoT connectivity. Application development
 supports the GCC environment.
 
-.. image:: img/rak5010-front-parts.jpg
-     :align: center
-     :alt: RAK5010-NRF52840
-
 Hardware
 ********