docs: add migration guide for v2 (#125)

Author: chenjiahan

docs/migrate-v1-to-v2.md

@@ -0,0 +1,155 @@
+# @rspack/dev-server Migration Guide (v1 -> v2)
+
+This guide covers breaking changes you need to handle when upgrading `@rspack/dev-server` from v1 to v2.
+
+## Breaking Changes
+
+### Drop Node 18 Support
+
+Node.js 18 is no longer supported in v2.
+
+The minimum supported Node.js version is now `^20.19.0 || >=22.12.0`.
+
+### Drop Rspack v1 Support
+
+`@rspack/dev-server` v2 is designed to work with Rspack v2. Rspack v1 is no longer supported in v2.
+
+### Upgraded `http-proxy-middleware` to v3
+
+`http-proxy-middleware` has been updated to v3, which has some breaking changes:
+
+- `proxy[].path` is removed. Use `pathFilter` (or `context`) instead.
+
+```diff
+{
+-  path: '/api',
++  pathFilter: '/api',
+}
+```
+
+- `logLevel` / `logProvider` are replaced by `logger`.
+
+```diff
+- logLevel: 'warn',
++ logger: console,
+```
+
+- `onProxyReq` / `onProxyRes` / `onProxyReqWs` are moved to `on: { ... }`.
+
+```diff
+- onProxyReq(proxyReq) {
+-   proxyReq.setHeader('x-from', 'dev-server');
+- },
++ on: {
++   proxyReq(proxyReq) {
++     proxyReq.setHeader('x-from', 'dev-server');
++   },
++ },
+```
+
+- `proxy[].bypass` is removed, use `pathFilter` or `router` instead.
+  - When `bypass` was used and that function returned a boolean, it would automatically result in a 404 request. This can’t be achieved in a similar way now, or, if it returned a string, you can do what was done in the example above.
+  - `bypass` also allowed sending data; this can no longer be done. If you really need to do it, you’d have to create a new route in the proxy that sends the same data, or alternatively create a new route on the main server and, following the example above, send the data you wanted.
+
+> Refer to the [http-proxy-middleware v3 migration guide](https://github.com/chimurai/http-proxy-middleware/blob/master/MIGRATION.md) for details.
+
+### Upgraded Express to v5
+
+`express`` has been updated to v5, see [Introducing Express v5: A New Era for the Node.js Framework](https://expressjs.com/2024/10/15/v5-release.html) for details.
+
+### Removed `spdy` support
+
+- `server.type: "spdy"` is no longer supported.
+- `server.options.spdy` is no longer supported.
+
+You can switch to `server.type: "http2"` or `"https"` instead.
+
+```js
+// Before
+export default {
+  devServer: {
+    server: {
+      type: 'spdy',
+      options: {
+        spdy: { protocols: ['h2', 'http/1.1'] },
+      },
+    },
+  },
+};
+
+// After
+export default {
+  devServer: {
+    server: {
+      type: 'http2', // or 'https'
+      options: {},
+    },
+  },
+};
+```
+
+### Removed SockJS support (`ws` only)
+
+In v2, the following SockJS options are no longer available:
+
+- `webSocketServer: "sockjs"`
+- `webSocketServer: { type: "sockjs" }`
+- `client.webSocketTransport: "sockjs"`
+- `webSocketServer.options.prefix` (use `path`)
+
+Also, SockJS bundle routes were removed.
+
+```js
+// Before
+export default {
+  devServer: {
+    webSocketServer: 'sockjs',
+    client: {
+      webSocketTransport: 'sockjs',
+    },
+  },
+};
+
+// After
+export default {
+  devServer: {
+    webSocketServer: 'ws',
+    client: {
+      webSocketTransport: 'ws',
+    },
+  },
+};
+```
+
+If you need a non-default transport/server, you can provide a custom implementation path instead of `'sockjs'`.
+
+### Removed `bonjour`
+
+Built-in Bonjour support is removed in v2.
+
+If Bonjour/ZeroConf broadcasting is still important for your workflow, consider integrating `bonjour-service` directly in your app code.
+
+### Renamed `webpack-dev-server-*` to `rspack-dev-server-*`
+
+If your setup still uses the legacy names, you can update them as follows:
+
+- Environment variables
+  - `WEBPACK_DEV_SERVER_BASE_PORT` -> `RSPACK_DEV_SERVER_BASE_PORT`
+  - `WEBPACK_DEV_SERVER_PORT_RETRY` -> `RSPACK_DEV_SERVER_PORT_RETRY`
+- URL query flags
+  - `webpack-dev-server-hot=false` -> `rspack-dev-server-hot=false`
+  - `webpack-dev-server-live-reload=false` -> `rspack-dev-server-live-reload=false`
+- Built-in routes
+  - `/webpack-dev-server/*` -> `/rspack-dev-server/*`
+
+```txt
+# Before
+http://localhost:8080/?webpack-dev-server-hot=false&webpack-dev-server-live-reload=false
+GET /webpack-dev-server/invalidate
+GET /webpack-dev-server/open-editor?fileName=...
+
+# After
+http://localhost:8080/?rspack-dev-server-hot=false&rspack-dev-server-live-reload=false
+GET /rspack-dev-server/invalidate
+GET /rspack-dev-server/open-editor?fileName=...
+```