add sphinx-build in tox

[ChoiByungWook]

Issue #, if available:
Build the sphinx docs similar to RTD build process: https://readthedocs.org/projects/sagemaker/builds/8577862/

Description of changes:
Added new env within tox which handles building the sphinx-docs similar to that of RTD.

Merge Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have updated the changelog with a description of my changes (if appropriate)
• I have updated any necessary documentation (if appropriate)

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[codecov-io]

Codecov Report

Merging #653 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #653   +/-   ##
=======================================
  Coverage   92.77%   92.77%           
=======================================
  Files          71       71           
  Lines        5465     5465           
=======================================
  Hits         5070     5070           
  Misses        395      395

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