Add a pytest-unittest translator to the conversion guide

[ESadek-MO]

Added a translation guide, as suggested in #5690 (comment).

Additionally, corrected myself; mocker has the same api as mock, and can be used as a context manager. In the example I based that judgement off, it was just decided it wasn't needed.

See https://pytest-mock.readthedocs.io/en/latest/usage.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.83%. Comparing base (7bca716) to head (c4d2043).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6213   +/-   ##
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  Coverage   89.83%   89.83%           
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  Files          88       88           
  Lines       23315    23315           
  Branches     4338     4338           
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  Hits        20945    20945           
  Misses       1644     1644           
  Partials      726      726           

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