Add official Next.js example with Mantine in Next.js repo

[nexxeln]

Why not add a template to the official examples directory of Next.js?

That way people can bootstrap a Next.js + Mantine Project with npx create-next-app -e with-mantine

[rtivital]

Because it will be much harder to update as it will require raising PR in Next.js main repo, it will not be synchronized with Mantine releases cycle.

[nexxeln]

Makes sense, thank you for all the amazing work!