doc: update CI content in onboarding doc

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

PR-URL: #8374
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>

doc/onboarding.md

@@ -106,13 +106,14 @@ onboarding session.
 
   * Continuous Integration (CI) Testing:
     * https://ci.nodejs.org/
-    * It is not automatically run. You need to start it manually.
+      * It is not automatically run. You need to start it manually.
     * Log in on CI is integrated with GitHub. Try to log in now!
     * You will be using `node-test-pull-request` most of the time. Go there now!
+      * Consider bookmarking it: https://ci.nodejs.org/job/node-test-pull-request/
     * To get to the form to start a job, click on `Build with Parameters`. (If you don't see it, that probably means you are not logged in!) Click it now!
     * To start CI testing from this screen, you need to fill in two elements on the form:
       * The `CERTIFY_SAFE` box should be checked. By checking it, you are indicating that you have reviewed the code you are about to test and you are confident that it does not contain any malicious code. (We don't want people hijacking our CI hosts to attack other hosts on the internet, for example!)
-      * The `PR_ID` box should be filled in with the number identifying the pull request containing the code you wish to test. For example, if the URL for the pull request is https://github.com/nodejs/node/issues/7006, then put `7006` in the `PR_ID`.
+      * The `PR_ID` box should be filled in with the number identifying the pull request containing the code you wish to test. For example, if the URL for the pull request is `https://github.com/nodejs/node/issues/7006`, then put `7006` in the `PR_ID`.
       * The remaining elements on the form are typically unchanged with the exception of `POST_STATUS_TO_PR`. Check that if you want a CI status indicator to be automatically inserted into the PR.