Add instructions in the pipeline editor for configuring jobs for MRs

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What does this MR do and why?

Add instructions in the pipeline editor for configuring jobs for MRs.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to any project.
2. Using a sidebar navigate to Build -> Pipeline editor.
3. Click the Help button under the Edit tab.
4. Review a result in the opened help drawer on the right.

Related to #501001 (closed)