docs: expand Security Gates with CI/CD examples

[farhanashrafdev]

Expand Security Gates documentation

Filled in the Security Gates stub (current-version/2-Process/2-3-Build/2-3-5-Security-Gates.md) with practical content.

What's added

• Explanation of what security gates are and why they matter
• Threshold configuration examples (what to block vs warn)
• Full CI/CD pipeline examples for both GitHub Actions and GitLab CI
• Coverage of all gate types: secrets, SAST, SCA, container, IaC
• Best practices: gradual rollout, handling false positives, emergency bypasses
• Sample gate output showing what failures look like
• Tools section (open-source and commercial) following the existing format

Why

The original file was just a TBD stub. Security gates are a core DevSecOps concept and the guideline needed concrete examples teams can actually use.

Checklist

• Follows existing document structure
• Uses - for lists (MD004)
• Tools listed alphabetically with footnote
• No trailing whitespace
• Tested locally with markdownlint