13a_learner: create docments

[warner-benjamin]

Add docments to 13a_learner.

See the style guide and contribution guide for more details (links will be added when created).

This requires advanced familiarity with the fastai learner, training loop, metrics, and callbacks.

Docments progress:

• [ ] Utils function
• [ ] Learner
• [ ] Serializing
• [ ] DataLoader aware to_detach
• [ ] Metrics Core
• [ ] Recorder
• [ ] Transfer learning
• [ ] TTA