chore: remove unnecessary vercel.json from docs (#6154)
Since project settings have been updated to remove the overrides, we can
now remove this `vercel.json` config.
I tested with different cache settings and both [`cache
hit`](https://vercel.com/vercel/turbo-site/FbmmE3JfV3XmKdg9TVcndbwzgR5A)
and [`cache
bypass`](https://vercel.com/vercel/turbo-site/TPumbZ8fSCKS2mSmMzytByf7gDka)
work.
