Draft for review: security/credentials

[StackScribe]

This is a draft to work on content. After https://github.com/jenkins-infra/jenkins.io/pull/4612 is merged, this PR should be integrated into _chapter.yml appropriately and be mentioned in the index.doc and managing-security.adoc files.

@daniel-beck @Wadeck We need your avuncular wisdom for the "Best Practices" material here. I noted a couple possible topics just to get you started but they may or may not be useful.