next.js build: building contentlayer prior to TSC or adding more clear docs

When following the official Getting Started example for Next.js, the app fails to build on CI or any environemnt where `.contentlayer` doesn't exist already.

This appears to be because TSC runs _prior_ to contentlayer, which errors out.

I'm not sure if this can be changed at all (to run a Next plugin prior to the TSC step). That would seem like the ideal solution, but otherwise, would be great to update the documentation & examples to modify the build step

```json
"scripts": {
"build": "contentlayer && next build"
}
```

I'm happy to make a PR to help out, just wanted to check if this is indeed an issue that others experience or if I'm really missing something basic

公式のGetting Startedサンプルとそのpackage.jsonのbuildスクリプトから始め、次に.contentlayerがない環境でCIの失敗を再現します。ドキュメントに記載されたbuildコマンドの前にContentlayerの実行が必要かどうかを確認します。サンプルが正常にビルドされ、ドキュメントに必要なコマンドまたは確認済みのサポート対象の動作が反映されていれば完了です。

索引モデルが issue の本文から書いたものです。