docs(elements): add link to full example in elements guide (#25219)

PR Close #25219

aio/content/examples/elements/stackblitz.json

@@ -0,0 +1,9 @@
+{
+  "description": "Angular Elements",
+  "files":[
+    "!**/*.d.ts",
+    "!**/*.js",
+    "!**/*.[1].*"
+  ],
+  "tags":["cookbook"]
+}

aio/content/guide/elements.md

@@ -158,3 +158,9 @@ For comparison, the demo shows both methods. One button adds the popup using the
 
   </code-pane>
 </code-tabs>
+
+<!--
+  StackBlitz transpiles code to ES5. The live example will not work without a polyfill.
+  Only offer a `.zip` to download for now.
+-->
+You can download the full code for the example <live-example downloadOnly>here</live-example>.