Rewrite docs #81

Great gem! 💎

The current readme feels very AI-generated and is quite verbose. The signal to noise ratio isn't very high.

Some other thoughts:

• Great that it supports generic use-cases (hotcell-core and client/server gems), and that the active storage stuff is layered on top.
• The set of failure codes is pretty long, are all of them needed?
  • Maybe just let the hotcell return a JSON response instead? And reduce failure modes to those related to crashes etc.