Remove usage.md from spec, point to client spec (#3918)

Fixes #3908

Signed-off-by: Hayden B <[email protected]>

specs/SIGNATURE_SPEC.md

@@ -127,13 +127,15 @@ Gyp4apdU7AXEwysEQIb034aPrTlpmxh90SnTZFs2DHOvCjCPPAmoWfuQUwPhSPRb
     - The `logIndex` is the index of the log entry in the transparency log
     - The `logID` is the SHA256 hash of the DER-encoded public key for the log at the time the entry was included in the log
 
-For instructions on using the `bundle` for verification, see [USAGE.md](../USAGE.md#verify-a-signature-was-added-to-the-transparency-log).
-
 * `rfc3161timestamp` string
 
   This OPTIONAL property contains a JSON formatted `RFC3161Timestamp` containing the timestamp response from a
   timestamp authority.
 
+### Verification
+
+See the [client specification on verification](https://github.com/sigstore/architecture-docs/blob/main/client-spec.md#4-verification).
+
 ## Storage
 
 `cosign` image signatures are stored in an OCI registry and are designed to make use of the existing specifications.