docs: Add use citation from publishing statistical models white paper

[matthewfeickert]

Description

Add use citation from Publishing statistical models: Getting the most out of particle physics experiments — the particle physics broader community white paper. The pyhf dev team were all coauthors on this paper.

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* Add use citation from 'Publishing statistical models: Getting the most out of particle physics experiments'
   - c.f. https://inspirehep.net/literature/1919763

[matthewfeickert]

Relevant RTD preview page: https://pyhf--1600.org.readthedocs.build/en/1600/citations.html#use-citations

[codecov]

Codecov Report

Merging #1600 (f612582) into master (4f18dbd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1600   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          63       63           
  Lines        4050     4050           
  Branches      576      576           
=======================================
  Hits         3957     3957           
  Misses         54       54           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.43% <ø> (ø)
unittests	97.48% <ø> (ø)

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