add info about PR reviews to the contributing docs file

[imatiach-msft]

Description

This PR adds some additional information about the code review process/PR approval to the contributing docs file.

Checklist

• I have added screenshots above for all UI changes.
• I have added e2e tests for all UI changes.
• Documentation was updated if it was needed.

[codecov-commenter]

Codecov Report

Merging #1472 (48dd87e) into main (e54ca69) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1472   +/-   ##
=======================================
  Coverage   89.48%   89.48%           
=======================================
  Files          84       84           
  Lines        3652     3652           
=======================================
  Hits         3268     3268           
  Misses        384      384           

Flag	Coverage Δ
unittests	89.48% <ø> (ø)

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[github-actions]

https://responsibleai.blob.core.windows.net/pullrequest/microsoft/responsible-ai-toolbox/ilmat/contributing-reviews/dashboard/index.html

[github-actions]

https://responsibleai.blob.core.windows.net/pullrequest/microsoft/responsible-ai-toolbox/ilmat/contributing-reviews/dashboard/index.html