EPIC: Placeholder Develop comprehensive, example-driven documentation for writing and using services and the messenger system

[desi]

For the services piece of this we should do this after the RPC service work.

The messenger system should come after the ADR for messenger delegation related - https://github.com/MetaMask/core/issues/4986 - aka: Here is how you use the wallet system from the UI

[mcmire]

Documentation of services is now covered by #5741. I think it'd probably be worth it to document the messenger system separately, but I will make a separate ticket for this.