[v4] firstChildRoute should return "index" route as first

[dimaMachina]

fixes #3606

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[github-actions]

📦 Next.js Bundle Analysis for swr-site

This analysis was generated by the Next.js Bundle Analysis action. 🤖

⚠️ Global Bundle Size Increased

Page	Size (compressed)
global	82.42 KB (2 B)

Details

The global bundle is the javascript bundle that loads alongside every page. It is in its own category because its impact is much higher - an increase to its size means that every page on your website loads slower, and a decrease means every page loads faster.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

If you want further insight into what is behind the changes, give @next/bundle-analyzer a try!

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📦 Next.js Bundle Analysis for docs

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌