docs: Add ss3L hepdata likelihood record

[matthewfeickert]

Description

Add HEPData record for published statistical model and observations for Search for squarks and gluinos in final states with same-sign leptons and jets using 139 fb−1 of data collected with the ATLAS detector.

ReadTheDocs build: https://pyhf.readthedocs.io/en/docs-add-ss3l-likelihoods-to-published/citations.html#published-likelihoods

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• Tests are passing
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* Add ATLAS ss3L HEPData record with patchset pallet (stat model and observations)
   - c.f. https://www.hepdata.net/record/ins1754675?version=3

[codecov]

Codecov Report

Merging #1133 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1133   +/-   ##
=======================================
  Coverage   97.02%   97.02%           
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  Files          62       62           
  Lines        3598     3598           
  Branches      519      519           
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  Hits         3491     3491           
  Misses         66       66           
  Partials       41       41           

Flag	Coverage Δ
#unittests	97.02% <ø> (ø)

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