doc: add commit formats for release blog posts

this commit enhances the guidelines to creating a release blog post, specifically by adding
a commit format that must be adhered to when creating a pull request on the website repository

Fixes: https://github.com/nodejs/node/issues/8629

PR-URL: https://github.com/nodejs/node/pull/8631


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@keybase.io>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>