docs: add ipc example (#3667)

examples/cli/ipc/README.md

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+# CLI: IPC
+
+The Unix socket to listen to (instead of a [host](../host-and-port/README.md)).
+
+## true
+
+Setting it to `true` will listen to a socket at `/your-os-temp-dir/webpack-dev-server.sock`:
+
+**webpack.config.js**
+
+```js
+module.exports = {
+  // ...
+  devServer: {
+    ipc: true,
+  },
+};
+```
+
+Usage via CLI:
+
+```console
+npx webpack serve --ipc
+```
+
+## string
+
+You can also listen to a different socket with:
+
+**webpack.config.js**
+
+```js
+const path = require("path");
+
+module.exports = {
+  // ...
+  devServer: {
+    ipc: path.join(__dirname, "my-socket.sock"),
+  },
+};
+```
+
+Usage via CLI:
+
+```console
+npx webpack serve --ipc ./my-socket.sock
+```
+
+## What Should Happen
+
+1. The script should listen to the socket provided.
+1. A proxy server should be created.
+1. Go to `http://localhost:8080/`, you should see the text on the page itself change to read `Success!`.

examples/cli/ipc/webpack.config.js

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+"use strict";
+
+const http = require("http");
+const httpProxy = require("http-proxy");
+// 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: {
+    webSocketServer: "ws",
+    onAfterSetupMiddleware: (server) => {
+      const proxyPort = 8080;
+      const proxyHost = "127.0.0.1";
+      const proxy = httpProxy.createProxyServer({
+        target: { socketPath: server.options.ipc },
+      });
+
+      const proxyServer = http.createServer((request, response) => {
+        // You can define here your custom logic to handle the request
+        // and then proxy the request.
+        proxy.web(request, response);
+      });
+
+      proxyServer.on("upgrade", (request, socket, head) => {
+        proxy.ws(request, socket, head);
+      });
+
+      proxyServer.listen(proxyPort, proxyHost);
+    },
+  },
+});

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

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