Update v3 docs info to point to current v4 docs

[kenmorse]

Now when going to the v3 Docs, there's a little information overlay in the lower left corner that redirects people to the v4 beta docs (https://v3.tailwindcss.com/docs/v4-beta). The text could now be something like "v4 Documentation" and the link, https://tailwindcss.com/docs.