docs: Add use citation from Spey paper

[matthewfeickert]

Description

Add use citation from 'Spey: smooth inference for reinterpretation studies' by @jackaraz.

• Use comes from spey-pyhf optional backend https://github.com/SpeysideHEP/spey-pyhf

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* Add use citation from 'Spey: smooth inference for reinterpretation
  studies'.
   - c.f. https://inspirehep.net/literature/2677291
   - Use comes from spey-pyhf optional backend
     https://github.com/SpeysideHEP/spey-pyhf

[matthewfeickert]

Relevant RTD build: https://pyhf--2258.org.readthedocs.build/en/2258/citations.html#use-citations

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (fb604e8) 98.30% compared to head (739ebfd) 98.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2258   +/-   ##
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  Coverage   98.30%   98.30%           
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  Files          69       69           
  Lines        4534     4534           
  Branches      802      802           
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  Hits         4457     4457           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	97.88% <ø> (ø)
doctest	61.09% <ø> (ø)
unittests-3.10	?
unittests-3.11	96.31% <ø> (ø)
unittests-3.8	96.33% <ø> (ø)
unittests-3.9	96.36% <ø> (ø)

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