Vary accept header for markdown

[icyJoseph]

Since this site is deployed to Vercel then:

• https://vercel.com/docs/cdn-cache#use-cases

Vercel's CDN already includes the Accept and Accept-Encoding headers as part of the cache key by default. You do not need to explicitly include these headers in your Vary header.

However, if you go to https://react.dev/reference/react and do:

fetch(window.location.href, {headers: {
    accept:'text/markdown'
}}).then(res=>res.text()).then(console.log)

And follow that with:

fetch(window.location.href).then(res=>res.text()).then(console.log)

You'll see that both return markdown. The network tab shows that the latter is read from disk. Reloading the page fetches fresh. This PR fixes that edge case, but in general, I think it is a good idea to be explicit about the Accept header being part of the cache key.

Deployment with this addition: https://react-dev-plum.vercel.app/learn - compare to what happens on react.dev

@rickhanlonii 🙏

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