docs: Add use citation from observable sim gen ML paper

[matthewfeickert]

Description

Add use citation from Systematic Evaluation of Generative Machine Learning Capability to Simulate Distributions of Observables at the Large Hadron Collider by Jan Gavranovič, Borut Paul Kerševan.

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• Tests are passing
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* Add use citation from 'Systematic Evaluation of Generative Machine
  Learning Capability to Simulate Distributions of Observables at the
  Large Hadron Collider'.
   - c.f. https://inspirehep.net/literature/2710627

[matthewfeickert]

Relevant RTD build: https://pyhf--2356.org.readthedocs.build/en/2356/citations.html#use-citations

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2eb13e8) 98.28% compared to head (9d38d88) 98.28%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2356   +/-   ##
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  Coverage   98.28%   98.28%           
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  Files          69       69           
  Lines        4538     4538           
  Branches      803      803           
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  Hits         4460     4460           
  Misses         45       45           
  Partials       33       33           

Flag	Coverage Δ
contrib	97.86% <ø> (ø)
doctest	60.70% <ø> (ø)
unittests-3.10	96.29% <ø> (ø)
unittests-3.11	96.29% <ø> (ø)
unittests-3.8	96.31% <ø> (ø)
unittests-3.9	96.34% <ø> (ø)

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