Docs for CAWG integration

Overview

We need to add docs on how to use CAWG identity, to be published on https://opensource.contentauthenticity.org/docs/. We also need to be sure the doc published to https://crates.io/crates/cawg-identity matches and corresponds to the other docs.

Code

https://github.com/contentauth/c2pa-rs/tree/main/cawg_identity

Related docs

This will need to be updated: https://opensource.contentauthenticity.org/docs/getting-started#identity

Review:

• https://opensource.contentauthenticity.org/docs/faqs/#how-can-i-prove-time-and-place-an-image-was-created-without-revealing-my-identity
• https://opensource.contentauthenticity.org/docs/manifest/assertions-actions#creative-work-assertion