Introduce parallel:matrix jobs like in GitHub Action CI/CD tutorial

[wdconinc]

The GitHub Actions CI/CD tutorial introduces matrices as a way to replicate similar jobs since it doesn't have extends (and since anchors are not all that common for this): https://hsf-training.github.io/hsf-training-cicd-github/09-the-matrix/index.html.

Gitlab has support for matrices too, https://docs.gitlab.com/ee/ci/yaml/#parallelmatrix (I use them frequently, even though it may not be needed here exactly). It could be useful to present it as an episode, maybe right after https://hsf-training.github.io/hsf-training-cicd/10-the-worlds-a-stage/index.html.

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This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.

[GuillermoFidalgo]

Will add this as a short episode after https://hsf-training.github.io/hsf-training-cicd/07-adding-ci-to-your-code/index.html