Add pubmed link to papers using tree models for mortality prediction

[tompollard]

I think it's interesting to see how common XGBoost (etc) papers are published using the approach that we've worked through in the lesson. Perhaps add a link to https://pubmed.ncbi.nlm.nih.gov/?term=xgboost+mortality+prediction at https://carpentries-incubator.github.io/machine-learning-trees-python/07-gradient-boosting/index.html