Docs: Add steps to preview Semgrep-based SAST using Latest template

What does this MR do?

Add how to use the Latest template to preview SAST changes, as suggested in gitlab-com/www-gitlab-com!109377 (comment 1059254153).

Note that the plan is really for this to matter only in 15.3—in 15.4, the stable template will be updated unless we discover showstopping issues during the preview period. I went back and forth a bit on how to present this content as a result.

Related issues

gitlab-com/www-gitlab-com!109377 (comment 1059254153)

Update `latest` SAST template to reflect upcomi... (#366242 - closed), part of SAST Deprecation: Analyzer consolidation and CI... (#352554 - closed)

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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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.
• 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

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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  • 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.
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• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.