Update links to new Format.JS docs site

[jahredhope]

https://formatjs.io/ now redirects to https://formatjs.github.io/

The redirect doesn't persist the path, so anyone using the current links will end up on the home page.

All pages we link to still appear to work by only replacing the domain name.

I've chosen not to update the links in the release notes as I'd prefer to treat them as immutable unless there is a real need.

The readme does get published in the npm package, so I could write a release note, but I've chosen not to. Happy to add one if people want.

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