Modernize XGBoost Python document. (#7468)

* Use sphinx gallery to integrate examples.
* Remove mock objects.
* Add dask doc inventory.

demo/guide-python/predict_first_ntree.py

@@ -1,3 +1,7 @@
+"""
+Demo for prediction using number of trees
+=========================================
+"""
 import os
 import numpy as np
 import xgboost as xgb