Add Guide and Documentation on using Custom Tree Models

[mmschlk]

We need an easy-to-find guide on how to add custom tree models that we are not nativlely supporting to the repository. This should be accessible in the documentation. I think the best way to do this would be a dedicated notebook file.

Custom trees can be used either as a dict or as a manually instantiated TreeModel objects. The relevant lines in validation are as follows:

shapiq/shapiq/explainer/tree/validation.py

Lines 31 to 32 in a999719

	if type(model).__name__ == "TreeModel":
	return model

shapiq/shapiq/explainer/tree/validation.py

Lines 34 to 35 in a999719

	if type(model).__name__ == "dict":
	return TreeModel(**model)