Draft: Add docs on how to enable archived Semgrep rules

What does this MR do?

Modifies the SAST analyser documentation to include information on how to enable archived Semgrep rules.

While most of this content is covered under Customize rulesets, the existing documentation is not tailored for this use case. As we're deprecating functionality that may be useful for customers, we should have explicit instructions on how to restore the old behaviour.

Related issues

• #351399 (closed)

Author's checklist

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Reviewer's checklist

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