doc: graduate WHATWG URL from Experimental

PR-URL: https://github.com/nodejs/node/pull/12710


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>