scripts: compliance: allow webp image files

WebP images are smaller for similar resolutions and quality while being
compatible with web browsers and other image processing tools.

For higher resolution images, be sure to limit the display size to make
the rendered pages look reasonable.

This was approved by Benjamin Cabé (@kartben) in this discussion thread:
https://github.com/zephyrproject-rtos/zephyr/pull/55488#issuecomment-1461792751



Signed-off-by: Jason Kridner <jkridner@beagleboard.org>