Lack of discussion of callback function in optimisation tutorial

[gustavdelius]

At https://docs.sciml.ai/Overview/stable/getting_started/fit_simulation/#Step-4:-Solve-the-Optimization-Problem the tutorial says: "This step will look very similar to the first optimization tutorial, except now we have a new cost function. Just like in that tutorial, we want to define a callback to monitor the solution process." However there is no discussion of callback functions in the first optimization tutorial. Presumably such a discussion was planned to be included but then wasn't because the example chosen in that first tutorial was too simple to make a callback function useful.

[Vaibhavdixit02]

Thanks for pointing that out, it should be better after #162

[nathanrboyer]

I elaborated in #197.