Major Issue: gaps in example usage documentation

[TobyB1702]

Whilst reviewing functionality testing i noticed the following areas don't have example usage in the docs, they are as follows:

• PBPEngine, ProbLinear, PBPNet - only examples in pbp_probabilistic_backpropagation_test.ipynb and not in offical docs
• BayesianConv2d
• VectorScaling
• negative_log_likelihood and brier_score
• decompose_regression_uncertainty
• enable_dropout and prune_model
• forward_select and classification_nll_criterion

Part of the JOSS review openjournals/joss-reviews#11055 (comment)