docs(backport): Use Plausible for page visit statistics

[matthewfeickert]

Description

Backport PR #2186

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• Tests are passing
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* Backport PR https://github.com/scikit-hep/pyhf/pull/2186
* Use the Scientific Python org's Plausible instance to collect page visit
  statistics for the pyhf docs by adding it to the html_js_files in the
  Sphinx config.
   - c.f. https://views.scientific-python.org/pyhf.readthedocs.io/

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release/v0.7.x@153b6e5). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@                Coverage Diff                @@
##             release/v0.7.x    #2188   +/-   ##
=================================================
  Coverage                  ?   98.30%           
=================================================
  Files                     ?       69           
  Lines                     ?     4532           
  Branches                  ?      801           
=================================================
  Hits                      ?     4455           
  Misses                    ?       45           
  Partials                  ?       32           

Flag	Coverage Δ
contrib	27.51% <0.00%> (?)
doctest	61.12% <0.00%> (?)
unittests-3.10	96.31% <0.00%> (?)
unittests-3.11	96.31% <0.00%> (?)
unittests-3.7	96.29% <0.00%> (?)
unittests-3.8	96.33% <0.00%> (?)
unittests-3.9	96.35% <0.00%> (?)

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