Converted example images to WebP

[radarhere]

#7962 mentioned concern over image file sizes in the documentation.

This converts the images under 'docs/example' to WebP, saving 121kb.

In doing so, I also found and corrected a typo in the im.thumbnail() filename in the tutorial and ImageOps.rst - it should be image_thumbnail, and the file is named that, but the script says imageops_thumbnail.