Add the need for successful pipelines to update security dashboard

What does this MR do?

Our docs should make it clear that Security Dashboards are only updated when a successfully completed pipeline, containing security scanner artifacts has been run on the default branch of a project.

Some development and/or deployment workflows may leverage manual blocking jobs, which can derail vulnerabilities from being considered part of main branch, potentially leading to duplicate vulnerabilities being presented within merge requests.

This MR adds that caveat, and offers a suggested workaround if manual jobs are present (Line 29).

I also suggest we point users to Scan Execution Policies in lieu of, or in addition to, scheduled pipelines.

Related issues

#330042 (closed)

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