Dedicated Docs for JWT and VC JWTs

[nitro-neal]

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.

[KendallWeihe]

Also relevant to this #134

@decentralgabe I think we need dedicated docs around JWT thangs, like we did for VCs and DIDs

[decentralgabe]

@nitro-neal do you believe the guidance in the spec is sufficient?

if not, would you like to add text that helps clarify this? and test vectors to match? please add to #168