Docs: Clarify baseline pipeline selection for MR metrics reports

What does this MR do?

Adds a "How baseline comparison works" subsection to the metrics reports documentation explaining:

• The SHA cascade fallback strategy used to find a baseline pipeline (target_sha → diff_base_sha → diff_start_sha)
• That no pipeline status filter is applied to the baseline selection
• The difference between merge request pipelines and branch pipelines for baseline resolution
• Guidance on ensuring a baseline is always available

Also cross-references the new section from the existing troubleshooting entry.

Related issues

Closes #593215

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