Document all @import options on functions and directives page

[MHJahanbakhsh]

Summary

• Document the full set of options that can be passed to @import (important, prefix(), source(), theme(), reference, and layer()) on the functions and directives page.
• Link out to the existing deeper guides for important/prefix, source detection, and individual stylesheet imports so this page is the central reference without duplicating those docs.

Fixes #2514

Test plan

• Preview the Functions and directives page and confirm the new @import options section renders correctly
• Click through the linked anchors (important flag, source base path, disabling Preflight, @reference)
• Spot-check that the example snippets match supported option syntax

[vercel]

@MHJahanbakhsh is attempting to deploy a commit to the Tailwind Labs Team on Vercel.

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

The previously reported full-entrypoint failure has been resolved by changing the theme(reference) example to import tailwindcss/theme.css, and no blocking failure remains.

_{Reviews (2): Last reviewed commit: "docs: use theme.css entrypoint for theme..." | Re-trigger Greptile}

[MHJahanbakhsh]

Addressed the Greptile feedback: the `theme(reference)` example now imports `tailwindcss/theme.css` instead of the full `tailwindcss` entrypoint, so copying the snippet no longer pulls in Preflight/utilities and breaks the build.