docs(higress): add security self-assessment for incubation

[EndlessSeeker]

Summary

Adds the Higress project-maintained Security Self-Assessment at the canonical path used for CNCF incubation due diligence.

• Original: https://github.com/higress-group/community/blob/main/docs/cncf/security-self-assessment.md
• Follows the CNCF TAG Security self-assessment outline
• Documents actors, trust boundaries, security functions, threat scenarios, development practices, and known gaps
• OpenSSF Best Practices status: Passing

Scope

This is a project self-assessment, not an independent audit or joint assessment. The content is copied from the project-maintained version approved in higress-group/higress#4177 and updated on 2026-08-06.

Checklist

• DCO signed
• Added as projects/higress/security-assessment/self-assessment.md
• Source and public evidence links included