docs(guides/styling): add note about Tailwind warning

[bitofbreeze]

When you first finish the Tailwind setup and start up the app, I was confused to see the following warning:

warn - No utility classes were detected in your source files. If this is unexpected, double-check the `content` option in your Tailwind CSS configuration.
warn - https://tailwindcss.com/docs/content-configuration

Turns out I just hadn't used Tailwind yet, so I added this note so others don't go around wondering what they did wrong.

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