Replace compromised polyfill.io with Cloudflare mirror

[ChristopherRabotin]

This change addresses the polyfill.io supply chain attack by replacing the compromised https://polyfill.io/v3/polyfill.min.js?features=es6 script with a safe alternative hosted by Cloudflare: https://cdnjs.cloudflare.com/polyfill/v3/polyfill.min.js?features=es6.

I verified the fix by:

1. Building the site locally using mkdocs build.
2. Running a local server with mkdocs serve.
3. Using Playwright to capture and inspect screenshots of the homepage and a math-heavy page.
4. Confirming that math notation (which depends on the polyfill/MathJax) renders correctly and no malicious login prompts appear.

I also ensured that reqs.txt is preserved, as it is essential for the build environment.

Fixes #42

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PR created automatically by Jules for task 2596601502515802253 started by @ChristopherRabotin

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[gemini-code-assist]

Code Review

This pull request updates the polyfill.io URL in mkdocs.yml to use cdnjs. The reviewer suggests removing the polyfill dependency entirely, as modern browsers natively support ES6 features, which improves performance and mitigates potential security risks.

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[github-actions]

Visit the preview URL for this PR (updated for commit 5282977):

https://nyx-space--pr43-fix-polyfill-exploit-c35zgp20.web.app

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