Make Kaffy integration more explicit

[florius0]

Describe the problem you're proposing to solve
Now we have to write a bunch of staff in various files. And it is not clear which URLs Kaffy uses, so putting it in scope with others things might break smth.

Describe the solution you'd like
Use the way of live dashboard does. So we have to do smth like this:

defmodule MyAppWeb.Router do
  
  import Kaffy.Router

  scope "/admin" do
    pipe_through :browser
    kaffy "/", repo: MyApp.Repo
  end
end

[zoedsoupe]

Yeah! It would be a great enhancement

[aesmail]

This might be a nice enhancement.
Will look into it for v0.10.