Ensure shortcode alt text gets displayed as markdown.

layouts/shortcodes/image-with-fallback.html

@@ -12,4 +12,4 @@
   <img src="{{ $url2 }}" alt="{{ $alt }}">
 </picture>
 -->
-<img src="{{ $url1 }}" alt="{{ $alt }}" onerror="this.onerror=null; this.src='{{ $url2 }}';">
+<img src="{{ $url1 }}" alt="{{ $alt | markdownify }}" onerror="this.onerror=null; this.src='{{ $url2 }}';">