feat: L1 -> L2 messaging

[enitrat]

This example will show how we can send a message to L2 from L1 and execute a function from a L2 contract annotated with #[l1_handler]. We will provide both Solidity code and Cairo code.

[julio4]

You can use this messaging workshop to get started.

[enitrat]

also the book now has a section on it, so I think we should just provide a very succinct view into how it's implemented with a simple example

[julio4]

I agree! See Starknet Book - L1-L2 messaging