doc: update CI content in onboarding doc

* add node-test-pull-request link
* minor formatting improvements

PR-URL: https://github.com/nodejs/node/pull/8374


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>