Created MAC governance hub docs

Property	Value
description	Published `doc/mac/` as the governance hub for the MAC ecosystem.
version	0.64.0
type	Create
related	MAC, Ecosystem, Repo Boundaries

• Published doc/mac/ as the curated governance hub for the MAC ecosystem.
• Preserved the existing conceptual MAC introduction while adding dedicated ecosystem, boundary, and workflow pages.
• Kept maintainer-only implementation detail in repo-local .codex and source repos instead of publishing it wholesale.