Restricted access to Jenkins CI undocumented

[MikeMcC399]

node/doc/contributing/pull-requests.md

Line 532 in 20e6a54

### Continuous integration testing

has a section that explains the CI flow in detail, and even has a link to https://ci.nodejs.org/ , which can't be viewed at all (at least for the most of us 😅) IMO it'd be nice to mention this briefly there as well

Originally posted by @efekrskl in #64256 (comment)

[MikeMcC399]

PR #64256 only addresses changes to doc/contributing/first-contributions.md regarding access to Jenkins CI by non-Collaborators.

Since there has apparently been a policy change to restrict access to Jenkins CI by non-Collaborators, documents in the doc/contributing directory need to be reviewed and updated.

References, such as in https://github.com/nodejs/node/blob/main/doc/contributing/security-release-process.md#release-day which says:

Lock down the CI to prevent public access to the CI machines, ping a member of @nodejs/jenkins-admins.

no longer accurately describe the current situation.

Furthermore, the error message displayed in this case, as noted in issue nodejs/build#4394, only says "Access Denied / is missing the Overall/Read permission" without conveying any further background information.