docs: Add general use citation from 'Branches of a Tree' paper

[matthewfeickert]

Description

Add general citation from Branches of a Tree: Taking Derivatives of Programs with Discrete and Branching Randomness in High Energy Physics by @makagan and @lukasheinrich.

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* Add general citation from 'Branches of a Tree: Taking Derivatives of
  Programs with Discrete and Branching Randomness in High Energy
  Physics'.
   - c.f. https://inspirehep.net/literature/2692444

[matthewfeickert]

Relvant RTD build: https://pyhf--2312.org.readthedocs.build/en/2312/citations.html#general-citations

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (65a1f6d) 98.30% compared to head (a320f49) 98.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2312   +/-   ##
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  Coverage   98.30%   98.30%           
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  Files          69       69           
  Lines        4536     4536           
  Branches      802      802           
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  Hits         4459     4459           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	97.88% <ø> (ø)
doctest	61.04% <ø> (ø)
unittests-3.10	96.31% <ø> (ø)
unittests-3.11	96.31% <ø> (ø)
unittests-3.8	96.34% <ø> (ø)
unittests-3.9	96.36% <ø> (ø)

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