Docs: Reframe SAST config steps, link to enforcement

What does this MR do?

• When we introduce SAST configuration steps, also link to the details of how to use Scan Execution Policies or Compliance Frameworks. I've encountered a number of customers who aren't aware of these options, which avoid the requirement of having to go individually to every project to enable SAST.
• Reframe "manually" to "in your CI/CD YAML". The existing framing, in my opinion, unnecessarily casts shade on the CI/CD template modification by describing it as a "manual" process. In reality, because the template is easy to include and the UI flow has certain limitations, CI/CD YAML-based configuration is not an option we should discourage.
• Introduce some of the context around how SAST scanning works before diving in to the steps and options.

Once this MR goes through, I can carry the changes forward to other Secure pages. It felt more efficient to review first before copying.

Related issues

N/A

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

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

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.