docs: fix API reference banner contrast in dark mode

The API reference banner kept its light background when links switched to light colors in dark mode. Add dark background, border, and text colors matching the note callout palette.

layouts/_partials/api-reference/manual-link.html

@@ -4,7 +4,7 @@
     {{ $manual := site.GetPage .manual }}
     {{ if and $manual (eq $manual.RelPermalink $.RelPermalink) }}
       <p
-        class="my-4 rounded border border-blue-200 bg-blue-50 p-4 text-sm text-blue-900"
+        class="my-4 rounded border border-blue-200 bg-blue-50 p-4 text-sm text-blue-900 dark:border-blue-600 dark:bg-blue-950 dark:text-blue-100"
       >
         <a href="{{ .url }}">Explore the {{ .title }} API {{ .version }} →</a>
       </p>