Update Tailwind installation section

At the moment Tailwind is added using the script html tag in the html header, see https://github.com/dwyl/phoenix-liveview-chat-example#15-tailwind-css-stylin

Phoenix 1.7 now includes Tailwind by default.
I think it makes sense to follow the default way to add Tailwind to Phoenix as people might be more used to it now.
We can follow the steps at https://github.com/dwyl/learn-tailwind/#part-2-tailwind-in-phoenix or https://tailwindcss.com/docs/guides/phoenix