Create Samples for LightGbm Ranker.

[zeahmed]

Need doc samples for lightgbm ranker.
Reference the samples in the xml docs for the two public lightgbm ranker methods.

machinelearning/src/Microsoft.ML.LightGBM/LightGbmCatalog.cs

Line 104 in f8e58dd

public static LightGbmRankingTrainer LightGbm(this RankingCatalog.RankingTrainers catalog,

[Ivanidzo4ka]

if you close this one, please also close this one: #776