[docs] Fix element.ref accessing warning on docs

[DiegoAndai]

Problem

Fix this warning (only visible on local):

To repro, simply checkout master, run docs and visit the homepage.

Solution

Remove the usage of legacyBehavior on the docs' Link component (source). This legacy behavior relies on accessing the child ref, causing the warning:

https://github.com/vercel/next.js/blob/38a6d0177eced9b56dfcc58f83662a659195af15/packages/next/src/client/link.tsx#L461

Here's the guide on removing this prop: vercel/next.js@489e65e.

I think we can remove it as the only thing we're doing on NextLinkComposed is adding an empty styled anchor, which Next's Link now does by default:

https://github.com/vercel/next.js/blob/38a6d0177eced9b56dfcc58f83662a659195af15/packages/next/src/client/link.tsx#L621-L623

So:

• For usages with legacyBehavior = true, the result is the same
• There shouldn't be any usages with legacyBehavior = false, as that would result in a nested anchor which is not valid HTML. I searched confirmed this in this repo, MUI X's, and Toolpad's.

Note: I updated the pages router examples accordingly.

[mui-bot]

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Bundle size report

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[Janpot]

Should be fine. I don't expect anyone cloning these examples with the intent of using the legacy behavior.