docs: add --web-socket-server (#3300)

Author: snitin315

examples/cli/web-server-socket/README.md

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+# CLI: web-socket-server
+
+To create a custom server implementation.
+
+## sockjs
+
+This mode uses [SockJS-node](https://github.com/sockjs/sockjs-node) as a server.
+
+```console
+npx webpack serve --web-socket-server sockjs --open-target
+```
+
+## ws
+
+This mode uses [ws](https://github.com/websockets/ws) as a server.
+
+```console
+npx webpack serve --web-socket-server ws --open-target
+```
+
+### What Should Happen
+
+1. The script should open `http://localhost:8080/` in your default browser.
+2. You should see the text on the page itself change to read `Success!`.

examples/cli/web-server-socket/app.js

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

examples/cli/web-server-socket/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',
+});