docs: Add use citation from Belle II in Moriond EW 2021 contribution

[matthewfeickert]

Description

Add pyhf use citation from Search for B⁺→ K⁺νν̅ decays using an inclusive tagging method at Belle II by the Belle II Collaboration in a contribution to the 2021 EW session of the 55th Rencontres de Moriond.

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• Tests are passing
• "WIP" removed from the title of the pull request
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* Add use citation from Belle II in Moriond EW 2021 contribution on B⁺→ K⁺νν̅ search
   - Contribution title: 'Search for B⁺→ K⁺νν̅ decays using an inclusive tagging method at Belle II'
   - c.f. https://inspirehep.net/literature/1863052

[codecov]

Codecov Report

Merging #1475 (e3863ee) into master (70f100c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1475   +/-   ##
=======================================
  Coverage   97.65%   97.65%           
=======================================
  Files          63       63           
  Lines        4006     4006           
  Branches      565      565           
=======================================
  Hits         3912     3912           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.51% <ø> (ø)
unittests	97.42% <ø> (ø)

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[matthewfeickert]

Relevant RTD build: https://pyhf--1475.org.readthedocs.build/en/1475/citations.html#use-in-publications