docs: no longer recommends polyfill.io

[SukkaW]

Steps for submitting a pull request:

• ALL pull requests should be made against the dev branch!
• Take a look at CONTRIBUTING.MD
• Reference any related issues.
• Provide a description of the proposed changes.
• PRs must pass Vint tests and add new Vader tests as applicable.
• Make sure to update the documentation in doc/vimwiki.txt if applicable,
  including the Changelog and Contributors sections.

vimwiki recommends polyfill.io for HTML export. 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 (polyfillpolyfill/polyfill-service#2835, polyfillpolyfill/polyfill-service#2838, alist-org/alist#6100), rendering the polyfill.io service extremely unstable. Since then, Fastly (Announcement) and Cloudflare (Announcement) has hosted their own instances of polyfill.io service.

[tinmarino]

Merged. Thank you @SukkaW.