docs: Add next-theme-ui

[simenandre]

next-theme-ui is a small library to ease using Theme UI with Next.js.

Replaces these imports:

import { Link } from '@theme-ui/components';
import { NavLink } from '@theme-ui/components';

with

import { Link } from 'next-theme-ui';
import { NavLink } from 'next-theme-ui';

<Link /> and <NavLink /> uses Link from the next/link package.

[vercel]

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🔍 Inspect: https://vercel.com/systemui/theme-ui/EfZzE4nacqBSWSyVV36rz26vxXvX
✅ Preview: https://theme-ui-git-fork-cobraz-add-next-theme-ui-systemui.vercel.app

[lachlanjc]

Thanks for the contribution!

[hasparus]

🚀 PR was released in v0.13.0 🚀