doc: porting_guide: encourage the use of webp for board images

It's already mentioned in the board.tmpl template file as well as in
documentation writing guidelines so fix this leftover and make sure
people contributing boards aim for webp format as it helps with file
size while supporting transparency.

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

Author: kartben

doc/hardware/porting/board_porting.rst

@@ -261,7 +261,7 @@ Your board directory should look like this:
    ├── board.cmake
    ├── CMakeLists.txt
    ├── doc
-   │   ├── plank.png
+   │   ├── plank.webp
    │   └── index.rst
    ├── Kconfig.plank
    ├── Kconfig.defconfig
@@ -299,7 +299,7 @@ The optional files are:
 - :file:`board.cmake`: used for :ref:`flash-and-debug-support`
 - :file:`CMakeLists.txt`: if you need to add additional source files to
   your build.
-- :file:`doc/index.rst`, :file:`doc/plank.png`: documentation for and a picture
+- :file:`doc/index.rst`, :file:`doc/plank.webp`: documentation for and a picture
   of your board. You only need this if you're :ref:`contributing-your-board` to
   Zephyr.
 - :file:`plank_<qualifiers>.yaml`: a YAML file with miscellaneous metadata used