Documentation: Add new exclude_paths Secret Detection variable

What does this MR do?

As part of #225273 (closed), we renamed a variable in the Secret Detection template. We need to add that variable as we missed adding it initially.

This MR adds SECRET_DETECTION_EXCLUDED_PATHS to the Secret Detection Docs

Parrent Issue: #225273 (closed) MR adding new variable: gitlab-org/security-products/analyzers/secrets!64 (merged)

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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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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
• Security reports checked/validated by a reviewer from the AppSec team