doc: remove broken link for keygen
Remove broken link to deprecated keygen element.
Replacing the link with something to the relevant part of the HTML spec might be OK, but I don't think it's necessary. People who need to know will find it and everyone else should be discouraged from using this.
Support for the keygen element was removed from Chrome in 2017 and removed from Firefox in 2019. We might consider deprecating and removing support ourselves, or at least marking the API as legacy.