es-toolkit: add news

assets/javascripts/news.json

@@ -1,4 +1,8 @@
 [
+  [
+    "2025-06-04",
+    "New documentation: <a href=\"/es_toolkit/\">es-toolkit</a>"
+  ],
   [
     "2025-05-28",
     "New documentation: <a href=\"/vertx/\">Vert.x</a>"