Handle Bikeshed 5.0.3

Using macros inside of autolinks previously worked only accidentally; I made it work *explicitly* in Bikeshed 5.0. I've since walked back that decision, and put it behind a pref in 5.0.3.

index.bs

@@ -77,7 +77,7 @@ Abstract: This specification defines an API enabling the creation and use of str
  specification also describes the functional model for WebAuthn conformant [=authenticators=], including their signature and
  [=attestation=] functionality.
 Boilerplate: omit conformance, omit feedback-header, omit abstract-header
-Markup Shorthands: css off, markdown on
+Markup Shorthands: css off, markdown on, macros-in-autolinks yes
 Implementation Report: https://www.w3.org/2020/12/webauthn-report.html
 </pre>