[Beta Docs] Add Turborepo to custom toolchains for monorepos.

[leerob]

For reviewers and future viewers: all of the Turborepo examples here are using React if you want to explore.

[github-actions]

Size Changes

📦 Next.js Bundle Analysis

This analysis was generated by the next.js bundle analysis action 🤖

One Page Changed Size

The following page changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/learn/start-a-new-react-project	49.23 KB (🟡 +16 B)	133.81 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

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

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 10% or more, there will be a red status indicator applied, indicating that special attention should be given to this.