[doc] Document update [skip ci] (#8784)

- Remove version specifics in cat demo.
- Remove aws yarn.
- Update faq.
- Stop mentioning MPI.
- Update sphinx inventory links.
- Fix typo.

doc/tutorials/index.rst

@@ -3,7 +3,7 @@ XGBoost Tutorials
 #################
 
 This section contains official tutorials inside XGBoost package.
-See `Awesome XGBoost <https://github.com/dmlc/xgboost/tree/master/demo>`_ for more resources.
+See `Awesome XGBoost <https://github.com/dmlc/xgboost/tree/master/demo>`_ for more resources. Also, don't miss the feature introductions in each package.
 
 .. toctree::
   :maxdepth: 1
@@ -11,7 +11,6 @@ See `Awesome XGBoost <https://github.com/dmlc/xgboost/tree/master/demo>`_ for mo
 
   model
   saving_model
-  Distributed XGBoost with AWS YARN <aws_yarn>
   kubernetes
   Distributed XGBoost with XGBoost4J-Spark <https://xgboost.readthedocs.io/en/latest/jvm/xgboost4j_spark_tutorial.html>
   Distributed XGBoost with XGBoost4J-Spark-GPU <https://xgboost.readthedocs.io/en/latest/jvm/xgboost4j_spark_gpu_tutorial.html>