Documentation: rule-pack synthesis using rules from multiple sources

This MR reintroduces the changes from !74893 (merged)

• SAST Handbook Page: http://main-ee-75234.178.62.207.141.nip.io/ee/user/application_security/sast/#customize-rulesets
• Secret Detection Handbook Page: http://main-ee-75234.178.62.207.141.nip.io/ee/user/application_security/secret_detection/#custom-rulesets

What does this MR do and why?

Provide some documentation for the changes introduced in #339614 (closed). The changes are not merged in yet so that this MR is effectively blocked by #339614 (closed). Therefore, this MR is currently marked as a draft. Once the documented feature is available in the product, we will remote the draft status.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.