Dco fix v2

[ihorman]

Description

This PR fixes the failing DCO check and updates documentation:

• CI: replace broken probot/dco@v2 reference with a Bash-based DCO verification workflow (.github/workflows/dco.yml).
• Docs: add Testing Strategy and Code Review Guidelines.
• Docs: restructure API documentation and add alternative patterns (REST/GraphQL/gRPC/AsyncAPI).
• Docs: add PlantUML diagram generation spec.

Commits in PR:

• 9b4275f ci: replace broken probot/dco with built-in bash DCO check
• bd130bc Added: Testing Strategy and Code Review Guidelines
• d3f2d22 Restructure API documentation and add alternative patterns
• 2ebda9d tiny updates related to REST API guidelines
• f2937a6 added spec for nice diagram generation in PlantUML format

Type of Change

• Documentation update
• New section/guide
• Bug fix (typo, broken link)
• Code example improvement
• Other: CI workflow fix (DCO check)

Checklist

• Commits are signed off (git commit -s)
• Changes follow DNA principles
• Code examples tested (N/A – docs only)
• Links are valid
• Self-review completed

Related Issues

Closes #7