docs: Add SAST False Positive Detection documentation

Add comprehensive documentation for the AI-powered SAST False Positive Detection feature introduced in GitLab 18.7.

Changes

• New file: doc/user/application_security/vulnerabilities/false_positive_detection.md - Complete guide covering:

  • How FP detection works
  • Automatic and manual triggers
  • Confidence scoring
  • Configuration options
  • Limitations and troubleshooting

• Updated: doc/user/application_security/vulnerabilities/_index.md - Added section on AI-powered false positive detection

• Updated: doc/user/application_security/sast/_index.md - Added subsection on reducing false positives with AI

• New file: doc/user/duo_agent_platform/flows/foundational_flows/sast_fp_detection.md - Foundational flow documentation

• Updated: doc/user/duo_agent_platform/flows/foundational_flows/_index.md - Added SAST FP Detection to list of available flows

Related Issues

• Relates to #581126 (Rollout issue for SAST FP detection)
• Relates to epic #17883 (closed) (Automatic Detection of SAST False Positives)

Documentation Structure

The documentation follows the recommended structure:

• Detailed guide in vulnerabilities section
• Overview in vulnerabilities index
• Link from SAST section
• Foundational flow documentation in Duo Agent Platform

Tier & Availability

• Tier: Ultimate
• Add-ons: Duo Core, Duo Pro, Duo Enterprise
• Status: Beta (18.7)
• Offering: GitLab.com, GitLab Self-Managed, GitLab Dedicated