Implement contact form

Something similar with what we had before on the old website. Subject+message should be sent to welcome@jsheroes.io

Used to be implemented with a server function in the old next.js codebase, via sendgrid: https://github.com/jsheroes/legacy-website/blob/master/pages/api/contact-form.js but we should see what's the easiest solution in the new format with the static site