Add Fathom analytics and build from committed example images

Netlify no longer runs every example sketch on each deploy. The
reference example images are committed in Reference/api_en/imgs, taken
from the live site, and fingerprints.json records the example code each
image was made from. A normal build copies them; it fails if an
example's code changed without its image being regenerated with
--images.

Also points fetch_processing_jar.sh at contribs.processing.org, since
the old py.processing.org download links return 404.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: SableRaf