text: Title: DOC: add user guide on lightgbm
  Body: Add user guide on how to use xorbits.lightgbm.
  
  You can add a rst file at doc/source/libraries/xorbits_train by following the code style on `doc/source/libraries/xorbits_data/numpy.rst` or `doc/source/libraries/xorbits_data/pandas.rst`
  