Create "Future of CI Pipeline Processing" design doc

What does this MR do and why?

Preview: ▶ https://main-ee-120867.docs.gitlab-review.app/ee/architecture/blueprints/ci_pipeline_processing/index.html ◀

In GitLab CI, we have some problems with the current architecture and behavior of the pipeline processing. These problems confuse users and make it hard to understand the pipeline processing, lead to unexpected and complex behaviors, and make it hard to implement new features. In this design doc, we will discuss the problems and define the goals of the pipeline processing.

We will not discuss proposals in this MR.

Most of these problems have been discussed before in &6788.

MR acceptance checklist

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