docs: Add use citation from simplified likelihoods ATLAS PUB note

[matthewfeickert]

Description

Add use citation from ATLAS PUB note Implementation of simplified likelihoods in HistFactory for searches for supersymmetry (ATL-PHYS-PUB-2021-038).

(Full disclosure, Giordon and Lukas were on the analysis team.)

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• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

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For the PR Assignees:

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* Add use citation from 'Implementation of simplified likelihoods in HistFactory for searches for supersymmetry'
   - ATLAS PUB note: ATL-PHYS-PUB-2021-038
   - c.f. https://cdsweb.cern.ch/record/2782654

[matthewfeickert]

Relevant RTD build: https://pyhf--1614.org.readthedocs.build/en/1614/citations.html#use-in-publications

[codecov]

Codecov Report

Merging #1614 (3850895) into master (4459510) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1614   +/-   ##
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  Coverage   97.70%   97.70%           
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  Files          63       63           
  Lines        4048     4048           
  Branches      576      576           
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  Hits         3955     3955           
  Misses         54       54           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.44% <ø> (ø)
unittests	97.48% <ø> (ø)

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