Update Code Quality troubleshooting

What does this MR do and why?

• Reorganize the troubleshooting section by categorizing the issue to be source vs target branch causes
• The error where Base pipeline codequality artifact not found was no longer in use and was removed in this MR: !137095 (diffs). We don't find it in the code anymore.
• Add a troubleshooting step on finding if pipelines ran on the base pipeline of the merge request (and hence if it generated a Code Quality report).

This change came from ticket #1 and ticket #2 (internal links) as the customers were checking on missing code quality findings in the merge request widget. cc @j.castillo

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