feat: add DEMO button to Doxygen header linking to WASM demo

Author: Johnson-Ogundeji

doxygen/header.html

@@ -50,6 +50,9 @@
    <!--BEGIN PROJECT_BRIEF--><div id="projectbrief">$projectbrief</div><!--END PROJECT_BRIEF-->
   </td>
   <!--END PROJECT_NAME-->
+  <td style="padding-left:16px;vertical-align:middle;">
+   <a href="$relpath^demo/index.html" style="display:inline-block;padding:6px 18px;background:#7c3aed;color:#fff;border-radius:6px;font-weight:600;font-size:14px;text-decoration:none;letter-spacing:0.5px;">DEMO</a>
+  </td>
   <!--BEGIN !PROJECT_NAME-->
    <!--BEGIN PROJECT_BRIEF-->
     <td>