Add optimistic extension to extensions index (#3474)

Author: lorenseanstewart

www/content/extensions/_index.md

@@ -130,6 +130,10 @@ htmx extensions are split into two categories:
       <td>{% markdown() %}  [dynamic-url](https://github.com/FumingPower3925/htmx-dynamic-url/blob/main/README.md)  {% end %}</td>
       <td>{% markdown() %}  Allows dynamic URL path templating using `{varName}` placeholders, resolved via configurable custom function or `window.` fallback. It does not rely on `hx-vals`. Useful when needing to perform requests to paths that depend on application state.  {% end %}</td>
     </tr>
+    <tr>
+      <td>{% markdown() %}  [optimistic]([https://github.com/FumingPower3925/htmx-dynamic-url](https://github.com/lorenseanstewart/hx-optimistic/blob/main/README.md)  {% end %}</td>
+      <td>{% markdown() %}  This extension provides a way to optimistically update the UI to increase perceived performance  {% end %}</td>
+    </tr>
   </tbody>
   <tbody>
     <tr><th scope="rowgroup" colspan="2">Data API</th></tr>