doc: require CI status indicator in PRs

Commits are often landed despite failing on one or more CI platforms.
Having a CI status indicator in the PR should make this less likely to
happen.

PR-URL: https://github.com/nodejs/node/pull/17151


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>