docs: Add general citation from MadJAX paper

[matthewfeickert]

Description

Add general citation from Differentiable Matrix Elements with MadJax by @lukasheinrich and @makagan. 🚀

The paper was also submitted to the Proceedings of the 20th International Workshop on Advanced Computing and Analysis Techniques in Physics Research (ACAT 2021), but as that doesn't seem to be published yet we'll just treat it as a standalone paper for now.

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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:

• Summarize commit messages into a comprehensive review of the PR

* Add general citation from 'Differentiable Matrix Elements with MadJax'.
   - c.f. https://inspirehep.net/literature/2040678

[codecov]

Codecov Report

Merging #1799 (b0c7050) into master (a8982e4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1799   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files          64       64           
  Lines        4270     4270           
  Branches      683      683           
=======================================
  Hits         4190     4190           
  Misses         46       46           
  Partials       34       34           

Flag	Coverage Δ
contrib	26.25% <ø> (ø)
doctest	60.58% <ø> (ø)
unittests	96.18% <ø> (ø)

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[matthewfeickert]

Relevant RTD build: https://pyhf--1799.org.readthedocs.build/en/1799/citations.html#general-citations