Add CI Pipeline Efficiency documentation

What does this MR do?

We want to share best practices with making CI pipelines efficient. This includes performance indicators, storage consumption, and more components involved.

Since my CI monitoring webcast touches parts in this direction, and I had the content still in my head, I went ahead with creating a first draft based on the storyline for the slides.

This MR adds two screenshots which showcase things which are otherwise hard to describe:

• DAG visualization with the critical path, compared to a long CI pipeline.
• Grafana dashboard for the Prometheus CI Exporter.

cc @jyavorska @marcel.amirault

Related issues

Closes #230611 (closed)

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