[Train] Document Callbacks

[matthewdeng]

Best practices for adding Ray Train Callbacks should be documented. When adding a new Callback, it should be clear what existing components can be leveraged or extended.

Some patterns to think about:

1. Class inheritance, metaclasses, mixins.
2. Composability of common functionality.
3. Adding arguments and using **info kwargs.
4. Ray Tune Callback interoperability ([Train][Tune] Unify Train and Tune Callbacks #21065).

This may also involve refactoring the current Callbacks to follow a more extensible pattern.