Remove most references to @babel/polyfill in Usage Guide and clarify polyfilling instructions.

[AndrewSouthpaw]

Closes https://github.com/babel/babel/issues/12401#issuecomment-734008135 (unfortunately filed in the wrong repo, didn't realize the website was a separate one).

Hope this helps! Tweaks are welcome, I was trying to keep with the tone I saw in the docs.

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[AndrewSouthpaw]

Fixed a merge conflict. 😄