boards/riscv/hifive_unmatched: add board image

This commit add the image of the SiFive HiFive Unmatched board to the
docs.

Signed-off-by: Filip Kokosinski <[email protected]>

Author: fkokosinski

boards/riscv/hifive_unmatched/doc/img/hifive_unmatched.png

@@ -9,6 +9,11 @@ Overview
 The HiFive Unmatched is a development board with a SiFive FU740-C000
 multi-core 64bit RISC-V SoC.
 
+.. image:: img/hifive_unmatched.png
+   :width: 650px
+   :align: center
+   :alt: SiFive HiFive Unmatched board
+
 Programming and debugging
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