Docs: Add accessibility-themed favicon

- Create SVG favicon combining Rails red with universal access symbol
- Add green checkmark representing testing/validation
- Update layout to include favicon in head section

Author: rayraycodes

docs_site/_layouts/default.html

@@ -6,6 +6,11 @@
   <meta name="viewport" content="width=device-width, initial-scale=1.0">
   <title>{% if page.title %}{{ page.title }} - {% endif %}{{ site.title }}</title>
   <meta name="description" content="{{ site.description }}">
+  
+  <!-- Favicon -->
+  <link rel="icon" type="image/svg+xml" href="{{ '/favicon.svg' | relative_url }}">
+  <link rel="alternate icon" href="{{ '/favicon.svg' | relative_url }}">
+  
   <style>
     * {
       margin: 0;