Add documentation for custom Secret Detection rule format

What does this MR do and why?

This MR adds documentation for the custom rule format used in Secret Detection, specifically explaining:

• The supported fields in custom rules, including GitLab-specific fields title and remediation
• How custom rules with matching IDs override extended rules
• Example TOML configurations showing proper rule format

This documentation helps users understand how to create and customize their Secret Detection rules, particularly when using the recently added title and remediation fields.

References

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• Support title and remediation text in secret de... (#511321 - closed) • Craig Smith • 17.9

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