Add note about before_script to SAST docs

What does this MR do?

Clarify that the pre-compilation strategy must be used when configuring a before_script for sast scan. With this MR I simply wanted to add the keyword so it's at least searchable but there may be a better place to raise this in the docs... 💭

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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