doc: make commit guidelines easier to reference

- Can now link to 'Commit Guidelines' from pull requests
- Breaks up commit requirements and recommendations

PR-URL: https://github.com/nodejs/node/pull/11732
Refs: https://github.com/nodejs/node/pull/11723#issuecomment-284556146


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>