Dedicated Docs for JWT and VC JWTs

Clarify:
https://www.w3.org/TR/vc-data-model/#jwt-decoding
Originally from https://github.com/TBD54566975/web5-js/pull/421/files

Clarify the language and vc jwt examples for these claims:

* If any of these steps fail, the function will throw a [Error] with a message indicating the nature of the failure:
* - exp MUST represent the expirationDate property, encoded as a UNIX timestamp (NumericDate).
* - iss MUST represent the issuer property of a verifiable credential or the holder property of a verifiable presentation.
* - nbf MUST represent issuanceDate, encoded as a UNIX timestamp (NumericDate).
* - jti MUST represent the id property of the verifiable credential or verifiable presentation.
* - sub MUST represent the id property contained in the credentialSubject.

* Once the verifications are successful, when recreating the VC data model object, this function will:
* - If exp is present, the UNIX timestamp MUST be converted to an [XMLSCHEMA11-2] date-time, and MUST be used to set the value of the expirationDate property of credentialSubject of the new JSON object.
* - If iss is present, the value MUST be used to set the issuer property of the new credential JSON object or the holder property of the new presentation JSON object.
* - If nbf is present, the UNIX timestamp MUST be converted to an [XMLSCHEMA11-2] date-time, and MUST be used to set the value of the issuanceDate property of the new JSON object.
* - If sub is present, the value MUST be used to set the value of the id property of credentialSubject of the new credential JSON object.
* - If jti is present, the value MUST be used to set the value of the id property of the new JSON object.