Add quantum_info.utils.total_variation_distance and improve documentation of quantum_info.utils

[renatomello]

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (93fa069) 100.00% compared to head (732384e) 100.00%.
Report is 65 commits behind head on master.

❗ Current head 732384e differs from pull request most recent head 73299a7. Consider uploading reports for the commit 73299a7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1175   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           68        68           
  Lines         9901      9924   +23     
=========================================
+ Hits          9901      9924   +23     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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