Document protobufs and structs

[spacey-sooty]

Resolves #2812

[PeterJohnson]

As currently written, this doesn't actually document why you would want to use them, how to use them, or how to create your own, which is the real crux of the issue. I would expect to see documentation of the pros/cons of each of these approaches (sendable, protobuf, struct), as well as code examples of how to use each, and code examples of how to create custom ones. This CD post could be used as a starting point for the "why" piece (and a piece of the "how" for built-in ones): https://www.chiefdelphi.com/t/introducing-monologue-annotation-based-telemetry-and-data-logging-for-java-teams/443917/5