Update cygnet web files, README, and wordnets.toml

- docs/: refresh index.html, relations.json, local.json from cygnet main
- docs/img/: add omw-logo.svg (cygnet now serves images from img/)
- etc/wordnets.toml: use english-wordnet-2025-plus.xml.gz
- README: rewrite with usage, build, and release instructions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>