doc: Add XGBoostProcessor

[tuliocasagrande]

Description of changes:
XGBoostProcessor was missing in the documentation.

1. https://sagemaker.readthedocs.io/en/stable/frameworks/xgboost/using_xgboost.html#sagemaker-xgboost-classes
   New bullet point:

2. https://sagemaker.readthedocs.io/en/stable/frameworks/xgboost/xgboost.html
   New class:

Testing done:
Build Sphinx docs locally.

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Codecov Report

Merging #3144 (ec51eda) into master (621e70e) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3144   +/-   ##
=======================================
  Coverage   89.61%   89.62%           
=======================================
  Files         200      200           
  Lines       17356    17356           
=======================================
+ Hits        15554    15555    +1     
+ Misses       1802     1801    -1     

Impacted Files	Coverage Δ
src/sagemaker/session.py	71.05% <0.00%> (+0.07%)	⬆️

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