docs: add AI/ML authoring taxonomy guide

[okason97]

Summary

• Adds docs/AI_ML_AUTHORING.md: a taxonomy/decision guide for AI/ML templates
  and extensions, so maintainers can decide template vs. extension for new
  AI/ML capabilities without re-deriving the reasoning each time.
• Follows epic Roadmap: AI/ML template and extension catalog #71's revised direction, not the original 14-entry proposal:
  chat/RAG/agents/guardrails stay FastAPI extensions on fastapi-starter
  (no new chat template type), MLOps starts with a single sklearn MVP,
  data-modality variants become extensions/examples rather than one template
  per framework × modality.
• Links the new guide from docs/AUTHORING.md and docs/MAINTENANCE_RUNBOOK.md.

Out of scope (tracked separately)

• templates.json categories/types, registry script wiring → Add registry mappings and CI profiles for AI/ML catalog #76
• docs/MLOPS_CONTRACT.md → Define the common MLOps template contract #83
• mlops-sklearn-starter, fastapi-ai-chat, fastapi-ai-guardrails,
  all-mlops-github-actions → Add mlops-sklearn-starter template #84, Add fastapi-ai-chat extension MVP #77, Add fastapi-ai-guardrails extension #82, Add all-mlops-github-actions extension #87

Closes #72