Fix introduction to scan execution policies

What does this MR do?

Fix introduction to scan execution policies:

• Fix the FLAG content formatting. The missing blank line meant the first paragraph was included in the FLAG's content.
• Fix a prior editing error that had removed the first portion of a sentence.
• Create a new "Jobs" topic because the information does not belong in the introduction.
• Move some prerequisite information in a "Prerequisites" block.
  • Note: The "Scan execution policy editor" topic is not exactly a task topic, but it was the best location (IMO) for what I've listed as prerequisites. This MR is intended as an MVC, to address some errors needing to be addressed ASAP.

Related issues

N/A

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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• If you're adding a new page, add the product tier badge under the H1 topic title.
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• ~"frontend"
• ~"backend"
• ~"type::bug"
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Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • 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.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.