docs: Add ATLAS top group probability model records through June 2021

[matthewfeickert]

Description

Add HEPData record for published statistical model and observations for the following ATLAS top physics results:

• Measurement of the tt̅ production cross-section in the lepton+jets channel at s√ =13 TeV with the ATLAS experiment
• Measurements of the inclusive and differential production cross sections of a top-quark-antiquark pair in association with a Z boson at s√ =13 TeV with the ATLAS detector
• Measurement of the tt̅tt̅ production cross section in pp collisions at s√ =13 TeV with the ATLAS detector

These models started to be published in Summer 2020 along with Phys. Lett. B 810 (2020) 135797, but were somehow missed by the pyhf team until very recently.

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• Tests are passing
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* Add HEPData records for published statistical models for ATLAS:
   - Measurement of the ttbar production cross-section in the lepton+jets channel at 13 TeV
      + c.f. https://www.hepdata.net/record/ins1802524
   - Measurement of ttZ cross sections in Run 2
      + c.f. https://www.hepdata.net/record/ins1853014
   - 4-top cross-section measurement
      + c.f. https://www.hepdata.net/record/ins1869695
* Add link ATLAS top physics public results page

[matthewfeickert]

@kratsg You mentioned on Twitter that the ATLAS top group has 3 results with published probability models (which is great!). What are the other two in addition to this one?

[matthewfeickert]

This is placed here to try to place it correctly in the chronology based on the "24 June 2020" on the top group public results page and the "Last updated on 2020-07-28 13:51" on the HEPData page.

[matthewfeickert]

Relevant RTD build page: https://pyhf--1681.org.readthedocs.build/en/1681/citations.html#published-statistical-models

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Codecov Report

Merging #1681 (7cde4da) into master (983ee58) will not change coverage.
The diff coverage is n/a.

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  Coverage   98.05%   98.05%           
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  Files          64       64           
  Lines        4213     4213           
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  Hits         4131     4131           
  Misses         49       49           
  Partials       33       33           

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contrib	25.30% <ø> (ø)
doctest	61.04% <ø> (ø)
unittests	96.36% <ø> (ø)

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[kratsg]

Can we add the 4-top search (https://www.hepdata.net/record/ins1869695) and the ttZ measurement (https://www.hepdata.net/record/ins1853014)?