Add examples of timestamps in the Predefined Variables reference

What does this MR do and why?

Simply adds a concrete example to make it more obvious what "the ISO 8601 format" actually means for CI_JOB_STARTED_AT and CI_PIPELINE_CREATED_AT (edit: and CI_COMMIT_TIMESTAMP).

Screenshots or screen recordings

N/A

How to set up and validate locally

Preview the edited markdown file and confirm that the output in the relevant rows of the table is appropriate.

MR acceptance checklist

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