docs: add onListening example (#3702)

Author: snitin315

examples/on-listening/README.md

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+# onListening
+
+Provides the ability to execute a custom function when webpack-dev-server starts listening for connections on a port.
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  // ...
+  devServer: {
+    onListening: (devServer) => {
+      const port = devServer.server.address().port;
+      console.log("Listening on port:", port);
+    },
+  },
+};
+```
+
+To run this example use the following command:
+
+```console
+npx webpack serve --open
+```
+
+## What Should Happen
+
+1. The script should open `https://localhost:8080/` in your default browser.
+2. You should see the text on the page itself change to read `Success!`.
+3. Check the terminal output, you should see `Listening on port: 8080` in the output.

examples/on-listening/app.js

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+"use strict";
+
+const target = document.querySelector("#target");
+
+target.classList.add("pass");
+target.innerHTML = "Success!";

examples/on-listening/webpack.config.js

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+"use strict";
+
+// our setup function adds behind-the-scenes bits to the config that all of our
+// examples need
+const { setup } = require("../util");
+
+module.exports = setup({
+  context: __dirname,
+  entry: "./app.js",
+  devServer: {
+    onListening: (devServer) => {
+      const port = devServer.server.address().port;
+      console.log("Listening on port:", port);
+    },
+  },
+});