Write a guide on contributing a new expression

[andygrove]

What is the problem the feature request solves?

We should write a detailed guide showing new contributors how to add support for a new expression in Comet. This would cover the Scala, Protobuf, and Rust changes, as well as testing approaches. We could create this based on a recent PR that added a new expression.

Describe the potential solution

No response

Additional context

No response

[tshauck]

Once #342 is approved/merged and if it's an appropriate example, I'd be happy to write this based off that work.

[parthchandra]

Also related to adding a new expression is adding a new aggregate expression (for example: 49bf503)

[andygrove]

And I suppose we also need a separate guide on adding cast expressions since there are specific test considerations