doc: update crypo Certficate class.

Update the dead link to <keygen> documentation.
Add a link to mozilla developper documentation because
W3C deleted the reference to this element.

Add a note to inform <keygen> element is deprecated since HTML 5.2.

PR-URL: https://github.com/nodejs/node/pull/18721
Fixes: https://github.com/nodejs/node/issues/18662


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>