docs: add model promotion evidence gates

[catcherintheroad-hub]

Created from review issue: #1377

Summary

• Add model promotion evidence gates that bind evaluated model artifacts to production deployments.
• Require immutable artifact identity across evaluation runs, model card versions, approval records, deploy manifests, rollback targets, and adapter/base model pairs.
• Add a model promotion evidence table and edge-case fixtures for mutable deploy aliases, approval drift, adapter/base mismatch, and complete promotion records.

Validation

• git diff --check
• Markdown fence/non-ASCII sanity check
• Reference URL checks for SLSA, OWASP LLM Top 10, and NIST AI RMF