Move Kamal-related configuration to ./kamal/*

[igor-alexandrov]

Here (https://twitter.com/motdotla/status/1738589409119961343) @motdotla suggested that Kamal's configuration is currently too Rails-centric. I agree that keeping the deploy configuration in ./config/ looks correct only in Rails. Yes, Kamal was originally introduced as a Rails tool, but now it has a wider audience. What do you think about using ./kamal/* as a default configuration folder for Kamal?

[dhh]

Don't think that adds any additional value even if you're not using Rails. That's still one more directory.