feature: readthedocs v2

[ahsan-z-khan]

Issue #, if available:

Description of changes:
Update readthedocs build version to v2
Testing done:
tox -e sphinx on py39 env

_______________________________________________________________ summary _______________________________________________________________
  sphinx: commands succeeded
  congratulations :)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-notebook-tests
• Commit ID: 5c4379a
• Result: SUCCEEDED
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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-slow-tests
• Commit ID: 5c4379a
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-local-mode-tests
• Commit ID: 5c4379a
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-python-sdk-unit-tests
• Commit ID: 5c4379a
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[codecov-commenter]

Codecov Report

Merging #3003 (5c4379a) into master (cbae73f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3003   +/-   ##
=======================================
  Coverage   89.89%   89.89%           
=======================================
  Files         196      196           
  Lines       16610    16610           
=======================================
  Hits        14932    14932           
  Misses       1678     1678           

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[mufaddal-rohawala]

merged as part of #3004