doc: sync security policy with nodejs.org

The Node.js security disclosure policy has diverged between the website and github:

• https://nodejs.org/en/security/
• https://github.com/nodejs/node/security/policy

The website is more recent and accurate, so sync the content from:

• https://github.com/nodejs/nodejs.org/blob/master/locale/en/security.md

• documentation is changed or added

• commit message follows commit guidelines