eigen3: add news entry

assets/javascripts/news.json

@@ -1,4 +1,8 @@
 [
+  [
+    "2022-03-31",
+    "New documentation: <a href=\"/eigen3/\">Eigen3</a>"
+  ],
   [
     "2022-02-21",
     "New documentation: <a href=\"/tailwindcss/\">Tailwind CSS</a>"