docs: add startCallback example (#3864)

Author: snitin315

examples/api/start-callback/README.md

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+# API: startCallback(callback)
+
+While it's recommended to run `webpack-dev-server` via the CLI, you may also choose to start a server via the API.
+
+This example demonstrates using `startCallback(callback)` method. It instructs `webpack-dev-server` instance to start the server and then run the callback function.
+
+```js
+const Webpack = require("webpack");
+const WebpackDevServer = require("webpack-dev-server");
+const webpackConfig = require("./webpack.config");
+
+const compiler = Webpack(webpackConfig);
+const devServerOptions = { ...webpackConfig.devServer };
+const server = new WebpackDevServer(devServerOptions, compiler);
+
+server.startCallback(() => {
+  console.log("Successfully started server on http://localhost:8080");
+});
+```
+
+Use the following command to run this example:
+
+```console
+node server.js
+```
+
+## What Should Happen
+
+1. Open `http://localhost:8080/` in your preferred browser.
+2. You should see the text on the page itself change to read `Success!`.
+3. You should see `Successfully started server on http://localhost:8080` in the terminal output.

examples/api/start-callback/app.js

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

examples/api/start-callback/server.js

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+"use strict";
+
+const Webpack = require("webpack");
+const WebpackDevServer = require("../../../lib/Server");
+const webpackConfig = require("./webpack.config");
+
+const compiler = Webpack(webpackConfig);
+const devServerOptions = { ...webpackConfig.devServer, open: true };
+const server = new WebpackDevServer(devServerOptions, compiler);
+
+server.startCallback(() => {
+  console.log("Successfully started server on http://localhost:8080");
+});

examples/api/start-callback/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",
+  output: {
+    filename: "bundle.js",
+  },
+  stats: {
+    colors: true,
+  },
+});