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

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

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

boards/seco/stm32f3_seco_d23/doc/index.rst

@@ -1,7 +1,4 @@
-.. _stm32f3_seco_d23_board:
-
-SECO SBC-3.5-PX30 (JUNO - D23) (STM32F302)
-##########################################
+.. zephyr:board:: stm32f3_seco_d23
 
 Overview
 ********
@@ -21,13 +18,6 @@ The audio functionalities are managed by the AudioCodec embedded in the RK-809
 PMIC. SBC-3.5-PX30 board is completed by a series of connectors with various
 interfaces (UART, SPI, I2C) managed by the microcontroller STM32F302VCT6.
 
-.. image:: img/stm32f3_seco_d23.jpg
-     :align: center
-     :alt: SECO SBC-3.5-PX30
-
-More information about the board can be found at the
-`SECO SBC-3.5-PX30 website`_.
-
 Hardware
 ********