Document callback definition and usage

[jefftyoder]

Issue:

A search of the dev docs and API for the "callback" shows the docs do not define callbacks, provide a clarifying example, or provide a short comment at the top of the callbacks.jl file discussing implementation. There is are references to callbacks in the docs when discussing other model components or features 1 2, but they do not explicitly demonstrate implementation.

There is a deprecated ClimateMachine.jl docs page which does define callbacks. Assuming callbacks were not radically changed, we could make use of the section here.

Proposed improvements:

• Add a docstring/module-level comment to callbacks.jl
• Add a section to the docs explaining what callbacks are, when they fire during the timestepping loop, and how to define a custom one
• Include a minimal working example