boards: shields: Update Renesas us159 da14531evz board photograph

Updated photograph of Renesas US159 DA14531EVZ board with a clearer
image and renamed file to match convention documented in the Board
Porting Guide (resolving issue with old image not appearing in
list of supported boards on Zephyr website).

Signed-off-by: Ian Morris <[email protected]>

Author: iandmorris

boards/shields/renesas_us159_da14531evz/doc/index.rst

@@ -9,7 +9,7 @@ Overview
 The Renesas US159 DA14531EVZ carries a `DA14531MOD`_ Bluetooth LE module
 in a `Digilent Pmod`_ |trade| form factor.
 
-.. figure:: us159-da14531evz-pmod.webp
+.. figure:: renesas_us159_da14531evz.webp
    :align: center
    :alt: Renesas US159 DA14531EVZ Pmod