Improve the documentation of API

[hbaniecki]

A few things come to mind:

• Description of explain()/explain_survival() does not say when to apply which function, while also all examples use explain()

• Add informative descriptions to surv_fi and surv_integrated_fi, e.g. what the function does

• Call x and explainer arguments "explainers"/"explainer objects" not "models"?

• Add references (articles, URLs) to model_performance() and metrics/loss functions https://modeloriented.github.io/survex/reference/model_performance.surv_explainer.html

• Add references (articles, URLs) to SurvLIME/SurvSHAP(t) methods

• Let's add an information which functions return the objects named in descriptions of plotting functions, e.g. "the surv_shap object returned by the predict_parts(..., type="SurvSHAP(t)") function"