Missing docs for lambda_black_box

The software provided under [lambda_black_box](https://github.com/MachineLearningLifeScience/poli/blob/dev/src/poli/core/lambda_black_box.py)

is not indexed or documented.

One has to know that this functionality exists.

We should provide it as an item in the docs overview and explain how to add a custom function through the provided wrappers.