Remove polyfill.io from the docs

[WassimBenzarti]

As announced everywhere, Cloudflare, Fastly, GitHub and several sources. Polyfill(.)io is a malicious CDN that needs to be avoided. Instead there are alternative options like Cloudflare and Fastly which provide the same features as the previous CDN.

This PR replaces all polyfill(.)io links to cloudflare's CDN.