docs(backport): Use Plausible for page visit statistics (#2188)

* Backport PR #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/

docs/conf.py

@@ -274,6 +274,10 @@ def setup(app):
 
 html_js_files = [
     'js/custom.js',
+    (
+        'https://views.scientific-python.org/js/plausible.js',
+        {"data-domain": "pyhf.readthedocs.io", "defer": "defer"},
+    ),
 ]
 
 # Add any extra paths that contain custom files (such as robots.txt or