Draft: Think of dialog to confirm whether to load JupyterLight

This is not perfect, in particular the `confirm(...)` is a blocking
call, which is not supposed to be used in a OnClik handler as an onclick
handler need to respond fast.

I think this need to be refactored into utilities functions, and
setTimeout(0, callback), and/or Promises.

That way the onClick finishes immediately and does not violate the
timeout limit.

As noted in the comment, the navigator api is experimental, so it woudl
be good to test properly. In Web Inspector  (Firefox and Chome at
least), it is possible to artificially throttle the connection to test
the logic.

jupyterlite_sphinx/jupyterlite_sphinx.js

@@ -52,13 +52,38 @@ window.jupyterliteConcatSearchParams = (iframeSrc, params) => {
   }
 };
 
+/*
+ * This is the callback that will be triggered when a user press the button
+ * to show this iframe.
+ *
+ */
 window.tryExamplesShowIframe = (
   examplesContainerId,
   iframeContainerId,
   iframeParentContainerId,
   iframeSrc,
   iframeHeight,
 ) => {
+  // this is an experimental API
+  // https://developer.mozilla.org/en-US/docs/Web/API/Navigator/connection
+  const networkinfo = navigator.connection || {};
+  const connectiontype = networkinfo.effectiveType || "";
+  if (
+    networkinfo.saveData == true ||
+    connectiontype.indexOf("2g") !== -1 ||
+    connectiontype.indexOf("3g") !== -1
+  ) {
+    console.log(
+      "We detected either a mettered or slow connection. Asking user if they are sure.",
+    );
+    const res = confirm(
+      "Opening JupyterLite may download several hundreds Mb of data. Do you wish to continue ?",
+    );
+    if (res === false) {
+      return;
+    }
+  }
+
   const examplesContainer = document.getElementById(examplesContainerId);
   const iframeParentContainer = document.getElementById(
     iframeParentContainerId,