Add Trainer to quicktour
This PR makes some edits to the `pipeline` section to focus less on all the tasks (and their definitions) it is capable of. Users probably only need a general representative idea of what it can do, and then they're more interested in diving into how to use the pipeline.

I also added a brief section on the `Trainer` here about the basic parameters it accepts and a small explanation of how to customize the training loop behavior to keep the quick tour short. I think the `Trainer` is pretty important to include since a lot of users use it for training, and we also use it in our finetune guides.
_The documentation is not available anymore as the PR was closed or merged._