Deploy `Phoenix` App to `Hetzner` using `Kamal`

Opening this issue as a reminder to capture/document how to deploy an `Elixir/Phoenix` App to `Hetzner`. 🚀 
Don't have time to focus on this right now, but will return to it on weekend when kids are asleep. 🛌 

# Todo

+ [ ] Research existing posts/videos on this topic. 🔍 
+ [ ] Synthesise existing knowledge 📝 
+ [ ] Attempt to replicate it for our `MVP` App. 🤞 
