Add docs for preliminary check for trust list certs

[crandmck]

Changes in this pull request

Add suggested sanity check for certs to be submitted for trust list.

Deploy preview: https://deploy-preview-188--cai-open-source.netlify.app/docs/verify-known-cert-list#checking-your-certificate .

Questions

To avoid errors, I had to add --args to the jq command here:

c2patool ./image.jpg trust --allowed_list ./cert.pem \
| jq --args '.manifests[].signature_info.issuer' "XYZ Inc."

Just want to confirm that's OK.

Also, you need all of these tools, which we should probably state as prerequisites to follow that section, right?

• jq I don't think this is installed by default (on macOS anyway). Maybe use brew install jq ?
• openssl Anyone working with certs probably has it, but for completeness...?
• C2PA Tool. Again, someone who's gotten this far probably has it already, but still.

Closes #187

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