docs: align tailwindcss 4 guide to the with-tailwind example (#10963)
### Description
Update the Tailwind v4 Guide to detect the classes used in the shared UI
package.
By default Tailwind detects classes in the current working directory.
To detect classes used in the `@repo/ui` packages we need to **either**
include add `@import 'tailwindcss';` in that package's `style.css`
**or** add the ui package as a source.
While the former was done in the [with-tailwind
example](https://github.com/vercel/turborepo/pull/10579), it can lead to
duplicate classes and
[conflicts](https://github.com/vercel/turborepo/pull/10657).
You can find Tailwind's documentation about this here:
https://tailwindcss.com/docs/detecting-classes-in-source-files#explicitly-registering-sources.
In order for Tailwind to pick up the classes used in the `@repo/ui`
package the web styles need