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

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.

doc/architecture/blueprints/ci_pipeline_processing/index.md

@@ -197,7 +197,7 @@ rollback_job:
 
 - `build_job` runs and fails.
 - `test_job` is skipped.
-- `rollback_job` is skipped because "skipped" is not a successful state in the DAG processing.
+- Even though `rollback_job` is `when: on_failure` and there is a failed job, it is skipped because the `needs` list has a "skipped" job.
 
 ### Problem 4: Canceled jobs