Merge pull request #33 from ucdavis/polyfill-security-fix

Replace polyfill.io CDN use due to supply chain attack

mkdocs.yml

@@ -91,7 +91,7 @@ extra_css:
   - stylesheets/extra.css
 extra_javascript:
   - javascripts/mathjax.js
-  - https://polyfill.io/v3/polyfill.min.js?features=es6
+  - https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6
   - https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
 
 theme: