[doc] Promote dask from experimental. [skip ci] (#7509)

doc/python/python_intro.rst

@@ -1,13 +1,23 @@
 ###########################
 Python Package Introduction
 ###########################
-This document gives a basic walkthrough of the xgboost package for Python.
+
+This document gives a basic walkthrough of the xgboost package for Python.  The Python
+package is consisted of 3 different interfaces, including native interface, scikit-learn
+interface and dask interface.  For introduction to dask interface please see
+:doc:`/tutorials/dask`.
 
 **List of other Helpful Links**
 
 * :doc:`/python/examples/index`
 * :doc:`Python API Reference <python_api>`
 
+**Contents**
+
+.. contents::
+  :backlinks: none
+  :local:
+
 Install XGBoost
 ---------------
 To install XGBoost, follow instructions in :doc:`/install`.
@@ -22,7 +32,8 @@ To verify your installation, run the following in Python:
 
 Data Interface
 --------------
-The XGBoost python module is able to load data from many types of different formats, including:
+The XGBoost python module is able to load data from many different types of data format,
+including:
 
 - NumPy 2D array
 - SciPy 2D sparse array