Docs: Aggregate AppSec troubleshooting

What does this MR do?

Support is seeing many tickets (internal) about

WARNING: … report.json: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1

There seemed to be no troubleshooting info about these different messages so far. However, they all seem to result from failed analyzer runs.

This docu update adds basic advice about where to find more specific errors/warnings and how to enable logging to find even more details.

The idea behind updating both application_security/index.md and job_artifacts.md with almost similar advice is that the latter is probably the "last resort". The AppSec-specific advice about SECURE_LOG_LEVEL seemed to be the common advice for all the other touched files.

Tasks to be discussed whether to include here or create more MRs

• Is a similar log level increase possible gl-code-quality-report.json?
  • or for any other CI feature that should create a report that GitLab needs to pick up as an artifact?
• The variable SECURE_LOG_LEVEL is explained on a handful of pages. Maybe deciding on one place, deduplicating all other copies and instead pointing to an SSoT would be useful?

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• [-] Update the permissions table.
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• [-] Add the product tier badge accordingly.
• [-] Add GitLab's version history note(s).
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Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
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