Document retry trigger job

What does this MR do and why?

For #367547 (closed)

Documentation for retrying trigger jobs. Some sections were edited to emphasize the differences between retrying a pipeline (which only retries failed and canceled jobs) and retrying a trigger job (which recreates a downstream pipeline).

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.