Expand alt-text descriptions of dataviz figures

[tobyhodges]

I think we should set aside some time to expand some of the alternative text descriptions, particularly for data visualisations. When we do, Writing alt text for data vizualisation provides some helpful guidance to follow.

### Tasks
- [x] episode 1: Huber loss plot
- [x] episode 2: penguin attributes pair plot
- [ ] episode 3: 03_regression_test_5_naive_baseline.png
- [ ] episode 3: 03_training_history_3_rmse_smaller_model.png
- [ ] episode 3: 03_training_history_3_rmse_early_stopping.png
- [ ] episode 3: 03_training_history_5_rmse_batchnorm.png
- [ ] episode 3: 03_regression_test_5_dropout_batchnorm.png
- [ ] episode 3: 03_scatter_plot_basel_model.png
- [ ] episode 4: 04_training_history_1.png
- [ ] episode 4: 04_training_history_loss_1.png
- [ ] episode 4: 04_dense_model_training_history.png
- [ ] episode 4: 04_training_history_2.png
- [ ] episode 4: 04_training_history_3.png
- [ ] episode 4: 04_training_history_loss_3.png
- [ ] bonus_material: 03_training_history_4_rmse_dropout.png

[svenvanderburg]

@tobyhodges I think this is solved for some alt texts with: https://github.com/carpentries-incubator/deep-learning-intro/pull/428, but that does not cover all of them right?

[tobyhodges]

Some, but certainly not all. This is something I am happy to work on independently or at the next sprint, but I would be happy to guide others on this too.

[tobyhodges]

I have updated the original comment above to include a list of the dataviz figures that I think could benefit from improved alt-text

[svenvanderburg]

Great! Feel free to pick it up. And otherwise we will decide who picks it up in the next sprint!