add documentation for verification key permissions

[barriebyron]

Add verification key info from Verification Key Permissions RFC to
https://docs.minaprotocol.com/zkapps/o1js/permissions#default-permissions

the default permission is derived from the protocol version:

let protocol_versions =
  let open Protocol_version in
  `Assoc [ ("txnVersion", `Int (transaction current)) ]

verification key permissions:

• (not really user facing) zkapp transactions embed the current zkapp version when specifying verification key permission updates
• (user facing) Verification key permissions configured by users are valid for the current zkapp version. If zkapps are upgraded, verification keys will need to be regenerated, so the old verification key can no longer be used. Thus, when the zkapp version changes, the verification key permission will fallback to Signature so that existing zkapps can have their verification keys updated using the private key for that account.