doc: split CONTRIBUTING.md

PR-URL: https://github.com/nodejs/node/pull/18271
Fixes: https://github.com/nodejs/node/issues/17842


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>