Self-host attribution avatar to drop cross-origin request

Replace the external matthewblode.com/avatar-sm.png with a local copy so
the footer avatar no longer makes a cross-origin round-trip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mblode

docs/img/avatar-sm.png

@@ -753,7 +753,7 @@ <h1>Form elements</h1>
       <p>
         Crafted by
         <a href="https://matthewblode.com" rel="author">
-          <img src="https://matthewblode.com/avatar-sm.png" alt="Matthew Blode" width="20" height="20" style="border-radius:9999px;vertical-align:middle" />
+          <img src="img/avatar-sm.png" alt="Matthew Blode" width="20" height="20" style="border-radius:9999px;vertical-align:middle" />
           Matthew Blode
         </a>
       </p>