Document how to use CI `dependencies` and `parallel:matrix` together

Stan Hurequested to merge
sh-gitlab-ci-document-dependencies-and-parallel-matrix into master

What does this MR do and why?

It wasn't clear how to use these keywords together.

This was brought up in https://forum.gitlab.com/t/ci-specifying-artifact-dependencies-when-using-parallel-matrix/45026.

MR acceptance checklist

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

Edited by Stan Hu

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