feat: add migrating to v2 solid-start guide (#1378)

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>
Co-authored-by: Birk Skyum <74932975+birkskyum@users.noreply.github.com>
Co-authored-by: Amir Hossein Hashemi <87268103+amirhhashemi@users.noreply.github.com>
Co-authored-by: Atila Fassina <atila@fassina.eu>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Derpius <49565664+Derpius@users.noreply.github.com>

Author: 8 people

src/routes/solid-start/data.json

@@ -3,6 +3,7 @@
 	"pages": [
 		"index.mdx",
 		"getting-started.mdx",
+		"migrating-from-v1.mdx",
 		"building-your-application",
 		"advanced",
 		"guides"

src/routes/solid-start/migrating-from-v1.mdx

@@ -0,0 +1,98 @@
+---
+title: Migrating from v1
+use_cases: >-
+  existing project, migration, upgrade
+tags:
+  - setup
+  - installation
+  - starter
+  - template
+  - quickstart
+  - init
+version: "2.0"
+description: >-
+  Migrate your SolidStart project from v1 to v2.
+---
+
+This is a migration guide of how to upgrade your v1 SolidStart app to our new v2 version.
+
+Please note that some third-party packages may not be compatible with v2 yet.
+
+## Migration steps
+
+### Update dependencies
+
+```package-install
+@solidjs/start@2.0.0-alpha.2 @solidjs/vite-plugin-nitro-2 vite@7
+```
+
+```package-remove
+vinxi
+```
+
+### Configuration files
+
+- Remove `app.config.ts`
+- Create `vite.config.ts`
+
+```tsx title="vite.config.ts"
+import { solidStart } from "@solidjs/start/config";
+import { defineConfig } from "vite";
+import { nitroV2Plugin } from "@solidjs/vite-plugin-nitro-2";
+
+export default defineConfig(() => {
+	return {
+		plugins: [
+			solidStart({
+				middleware: "./src/middleware/index.ts",
+			}),
+			nitroV2Plugin(),
+		],
+	};
+});
+```
+
+Compile-time environment variables are now handled by Vite's environment API.
+
+```tsx title="vite.config.ts"
+// ...
+export default defineConfig(({ mode }) => {
+	const env = loadEnv(mode, process.cwd(), "");
+
+	return {
+		// ...
+		environments: {
+			ssr: {
+				define: {
+					"process.env.DATABASE_URL": JSON.stringify(env.DATABASE_URL),
+				},
+			},
+		},
+	};
+});
+```
+
+Update the build/dev commands to use native Vite instead of vinxi.
+
+```json
+"scripts": {
+  "dev": "vite dev",
+  "build": "vite build",
+  "start": "vite preview"
+}
+```
+
+### Environment types
+
+Only the `types` entry is new in v2. Everything else can remain unchanged.
+
+```json
+"compilerOptions": {
+  "types": ["@solidjs/start/env"]
+}
+```
+
+## Server runtime helpers
+
+- Replace all imports from `vinxi/http` with `@solidjs/start/http`
+- Optional: update the middleware syntax to the newer [H3 syntax](https://h3.dev/guide/basics/middleware)