Use relative link to user guide of matching version

Author: marcphilipp

documentation/src/javadoc/junit-overview.html

@@ -21,6 +21,6 @@
     </dd>
 </dl>
 
-<p>Already consulted the <a href="https://junit.org/junit5/docs/current/user-guide">JUnit 5 User Guide</a>?</p>
+<p>Already consulted the <a href="../user-guide/index.html">JUnit 5 User Guide</a>?</p>
 
 </body>