Docs: Advise to troubleshoot DS errors first with SECURE_LOG_LEVEL

What does this MR do?

Our /user/application_security/dependency_scanning page didn't highlight the importance of that CI variable in debugging. This MR attempts to rectify that by inserting it as the first troubleshooting section.

Related issues

Discovered while replying to this internal ticket

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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  • Style Guide.
• [-] If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
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• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
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    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
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