docs(example): add lazy example to docs

Author: keithamus

examples/index.html

@@ -8,5 +8,9 @@
   <include-fragment src="./pull.html">Loading</include-fragment>
   <!-- <script type="module" src="../dist/index.js"></script> -->
   <script type="module" src="https://unpkg.com/@github/include-fragment-element@latest?module"></script>
+  <details>
+    <summary>Click to unfold a lazy include-fragment</summary>
+    <include-fragment src="./pull.html" loading="lazy">Loading</include-fragment>
+  </details>
 </body>
 </html>