docs: replace `polyfill.io`

[SukkaW]

https://react.semantic-ui.com uses polyfill.io. The PR replaces polyfill.io with cdnjs.cloudflare.com/polyfill.

---

polyfill.io was acquired by a China-based CDN company "Funnull", see the announcement from the polyfill.io domain owner's Twitter and https://github.com/polyfillpolyfill/polyfill-service/issues/2834. Despite Funnull's claims of operating in the United States, the predominance of Simplified Chinese on its website suggests otherwise, and it turns out that "Funnull" is notorious for providing service for the betting and pornography industries.

The original creator of the polyfill.io has voiced his concern on Twitter. And since the acquisition, numerous issues have emerged (https://github.com/polyfillpolyfill/polyfill-service/issues/2835, https://github.com/polyfillpolyfill/polyfill-service/issues/2838, https://github.com/alist-org/alist/issues/6100), rendering the polyfill.io service extremely unstable. Since then, Fastly (Announcement) and Cloudflare (Announcement) has hosted their own instances of polyfill.io service.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.50%. Comparing base (21cce97) to head (74d221b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4467 +/- ## ======================================= Coverage 99.50% 99.50% ======================================= Files 186 186 Lines 3465 3465 ======================================= Hits 3448 3448 Misses 17 17 ```

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