Adding a guide to training loops and callbacks

[BradLarson]

This adds a guide for our experimental training loop abstraction, the built-in callbacks, and how to write a custom callback.

[sgugger]

Oh, thanks for the shout-out!

[BradLarson]

@sgugger - Well, it is your design, so to do anything else would at best be disingenuous. I also made sure to link to your awesome WordSeg analysis in our new guides to hopefully raise the visibility of that as well.