Add xk6-webtransport extension for v0.49

Documentation release for v0.49 happened after this PR #1490 was created. This applies the same change to v0.49 as was provided by #1490.

Signed-off-by: Paul Balogh <[email protected]>

Author: javaducky

docs/sources/v0.49.x/extensions/explore.md

@@ -335,6 +335,10 @@ Use the table to explore the many extensions. Questions? Feel free to join the d
         <h4>xk6-wamp</h4>
         <p>Add support for WAMP protocol</p>
     </a>
+    <a href="https://github.com/kelseyaubrecht/xk6-webtransport" target="_blank" class="nav-cards__item nav-cards__item--guide">
+        <h4>xk6-webtransport</h4>
+        <p>Add support for webtransport protocol</p>
+    </a>
     <a href="https://github.com/szkiba/xk6-yaml" target="_blank" class="nav-cards__item nav-cards__item--guide">
         <h4>xk6-yaml</h4>
         <p>Encode and decode YAML values</p>