Docs: Add instructions for using an older SAST template or image version

What does this MR do?

What:

• Add instructions for using a previous version of the SAST CI/CD template if you run into errors caused by a recent change
• Add a troubleshooting section that highlights the fact that you can use pinning to work around analyzer issues

Why:

• We want to make it easier for users (and GitLab team members supporting users) to immediately unblock themselves if we push a change to the GitLab-managed CI/CD template that breaks their workflow.
  • This could happen intentionally, in an intentional announced breaking change, or it could be the result of an accident.
  • Having an immediate self-service workaround gets users back in action without waiting for a Support response or a code change. (Recall that code changes also may take days to land in .com, or longer in Self-Managed.)

Related issues

• We've recently documented a similar solution as a way to preview upcoming changes by using the Latest template.
• We discussed doing exactly this in !97216 (comment 1101148606) as part of reducing risk of a change.

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 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.