Add documentation for the polyfill packages

[xabbuh]

Thanks to the different polyfill packages many PHP functions that have been introduced in recent versions can also be used in earlier PHP versions. We should create a section that explains what a polyfill is and which versions are now avaiable when using each polyfill package.

[wouterj]

👍 I was thinking about this as well when creating #6044.

Maybe just document it as a Symfony component in the components section?

[javiereguiluz]

Fixed by #9846.