feat(site): add Demo link to header navigation

Author: raphaelmansuy

site/src/components/SocialIcons.astro

@@ -10,6 +10,7 @@
     <a href="/getting-started/quick-start-python/" class="nav-link">Docs</a>
     <a href="/guides/rag-integration/" class="nav-link">Guides</a>
     <a href="/benchmark/results/" class="nav-link">Benchmark</a>
+    <a href="/demo/" class="nav-link nav-link--demo">Demo</a>
   </nav>
   <div class="social-actions">
     <a
@@ -58,6 +59,15 @@
     background-color: var(--sl-color-gray-6, rgba(255,255,255,0.08));
   }
 
+  .nav-link--demo {
+    color: var(--sl-color-accent, #4F46E5);
+  }
+
+  .nav-link--demo:hover {
+    color: #fff;
+    background-color: var(--sl-color-accent, #4F46E5);
+  }
+
   .social-actions {
     display: flex;
     align-items: center;