feat: add form to better showcase how email is send in nextjs end to end

[lior-amsalem]

Adding this example will help people to understand how email can be send from the UI all the way to the server side.

I've also wrote an article how to intergrate supabase, nextjs and resend and decide to try and contribute here as well

otherwise, people still can just use some part of the code if they want to.

i've also improve error msging as it was not 100% clear.

[gabrielmfern]

With our examples, we prefer to take the approach of keeping things simple and mostly trying to showcase the code that users are required to add. Also, most of the code added in this is Next.js-related, rather than Resend-related, so I don't think it really helps people using it.

That being said, thank you for contributing, but we're not going to take this one :)