Docs for MR pipeline rule interactions

What does this MR do?

Adds documentation on how to set up pipelines for merge requests in slightly more complicated scenarios.

Review: https://gitlab-review-57287-docu-jia93n.ce.gitlab-review.app/help/ci/merge_request_pipelines/index.md#having-all-jobs-always-run-except-for-a-few

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57287

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards

/label ~Documentation