Document that assets in public-testfiles repo were signed with untrusted test cert in C2PA Tool

[crandmck]

The claims on assets in https://c2pa.org/public-testfiles/ were signed with the test cert included with C2PA Tool, which is not on the known cert list https://opensource.contentauthenticity.org/docs/verify-known-cert-list. Clarify that in the docs.

cf https://discord.com/channels/983153151341371422/986451330992320512/1249754541059211334

Alternatively, we could:

• Sign them using the trust option per the c2patool docs Probably dont want to do this per Maurice.
• Create a new repo for CAI test images, separate from the c2pa.org repo with examples that have trusted claims.

[crandmck]

Fixed by #182.