Update AI governance overview for OpenMetadata 2.0

[harshach]

Summary

• replace the future-looking AI Governance Initiative callout with released OpenMetadata 2.0 language
• list the governance frameworks, controls, compliance evidence, policy evaluation, approval workflows, and audit reports now represented in the 2.0 standards
• replace the roadmap issue link with the 2.0 AI Governance schemas and APIs reference

Validation

• internal documentation target and section anchor verified
• MkDocs production build passes
• git diff --check passes