Adding section to generate SSL certificats with Traefik

[guillaumebriday]

Heavily inspired from https://github.com/basecamp/kamal/discussions/112

I think it could be nice to have proper example of how to use built in traefik mechanism to generate SSL certificats. 👍

Also, here is a blog post about how to use from scratch: https://guillaumebriday.fr/how-to-deploy-rails-with-kamal-and-ssl-certificate-on-vps