chore: add link to joint-welcome-session to welcome-events

Author: nakomochi

src/pages/welcome-events.astro

@@ -47,9 +47,13 @@ import { Picture } from "astro:assets";
         </ul>
       </section>
       <section id="joint-welcome-event">
-        <h3>エンジニア系サークル合同新歓</h3>
+        <h3>東大エンジニア系サークル合同新歓</h3>
         <ul>
           <li>日時: 4/9 (木), 4/15 (水)</li>
+          <li>
+            <a href="/articles/2026/03-30_joint-welcome-session"
+              >詳細はこちら</a>
+          </li>
         </ul>
       </section>
       <div class="divider">学習系イベント</div>