Improve Leaf Node Assignment documentation

[maurever]

Add an example of how to call leaf node assignment and explain the results. On the algo page, there is only a short description, see the DRF page for example:

https://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-science/drf.html#leaf-node-assignment

I found example here (https://docs.h2o.ai/h2o/latest-stable/h2o-docs/performance-and-prediction.html?#predicting-leaf-node-assignment), so in minimal we should add this link into every tree algorithm.