Remove compromised polyfill.io dependency

polyfill.io was acquired by a malicious actor in 2024 and used to inject
malicious JS into visitor browsers. The ES6 polyfill is unnecessary for
modern browsers and was only included as boilerplate from the MkDocs
Material MathJax setup guide.

Manually reviewed by PBJ
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

mkdocs.yml

@@ -143,7 +143,6 @@ markdown_extensions:
 
 extra_javascript:
   - scripts/mathjax.js
-  - https://polyfill.io/v3/polyfill.min.js?features=es6
   - https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js
 
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