[docs-infra] Add mdx support to vale lint

[oliviertassinari]

A quick fix for a regression that I noticed while fixing mui/material-ui#44277. We were no longer linting the docs. This got broken with #538.

Ideally, we could use errata-ai/vale#841, but it's not there. Instead, there are a bunch of examples online on how this can be done https://github.com/search?q=%5Bformats%5D+mdx+%3D+md&type=code.

We will need to propagate this to the other repositories. But unclear if we should do it just yet, we might want to wait.
Point 20. of #588 doesn't make it super clear that MDX is compatible with a great UX for developers. I imagine that it's solvable, but it's it's not it seems to mean that we need to remove MDX and go back to the previous approach that yields better performance.

[mui-bot]

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[vladmoroz]

Point 20. of #588 doesn't make it super clear that MDX is compatible with a great UX for developers. I imagine that it's solvable, but it's it's not, it seems to mean that we need to remove MDX and go back to the previous approach that yields better performance.

I'll look into it, seems that we'd need to lean in more into server components there to fix it instead of passing around the code snippets in React context.

Compressed it's about the same though, comparing the dialog pages:

Neither old or new docs get stellar ratings on PageSpeed Insights too:

We should be able to do better though with just a docs website.

[oliviertassinari]

This is kind of magic, it lint mdx too, it doesn't need to be:

Suggested change

	[*.md]
	[*.{md,mdx}]

so I didn't change it 🤷‍♂️

[michaldudak]

Point 20. of #588 doesn't make it super clear that MDX is compatible with a great UX for developers.

This isn't caused by MDX, but the Next.js App Router.