Write a proper specification for external chain interactions and CCTX state transitions

[lumtis]

Describe the Issue The current documentation is lacking info about how ZetaChain and external chain interact (on the protocol level, not user interactions) and how the state of a cctx is transitioned in consequence.

This makes onboarding into the source code harder. A better protocol documentation would also be better when extending the functionalities, having an overview of the current design to define a specification for extension.